Fix serialization of grants for REST API

[atoulme]

Description

This PR fixes an issue encountered when trying to retrieve grant information through a REST API.
The REST API is not documented, yet exists.

Refers/Fixes

Steps to reproduce:

• Go to https://gitcoin.co/api/v0.1/grants/
  Expected: this should render a list of grants in the system as JSON objects.
  Actual: the following is shown:

active: "error",
code: 500,
nav: "home",
title: "Error 500"
}```
Locally, go to `http://localhost:8000/api/v0.1/grants/`, showing the full stacktrace:
```172.20.0.1 - - [31/Mar/2020 23:24:26] "GET /api/v0.1/grants/ HTTP/1.1" 500 -
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/django/contrib/staticfiles/handlers.py", line 65, in __call__
    return self.application(environ, start_response)
  File "/usr/local/lib/python3.6/dist-packages/django/core/handlers/wsgi.py", line 141, in __call__
    response = self.get_response(request)
  File "/usr/local/lib/python3.6/dist-packages/django/core/handlers/base.py", line 75, in get_response
    response = self._middleware_chain(request)
  File "/usr/local/lib/python3.6/dist-packages/django/core/handlers/exception.py", line 36, in inner
    response = response_for_exception(request, exc)
  File "/usr/local/lib/python3.6/dist-packages/django/core/handlers/exception.py", line 90, in response_for_exception
    response = handle_uncaught_exception(request, get_resolver(get_urlconf()), sys.exc_info())
  File "/usr/local/lib/python3.6/dist-packages/django/core/handlers/exception.py", line 125, in handle_uncaught_exception
    return debug.technical_500_response(request, *exc_info)
  File "/usr/local/lib/python3.6/dist-packages/django_extensions/management/technical_response.py", line 37, in null_technical_500_response
    six.reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.6/dist-packages/six.py", line 702, in reraise
    raise value.with_traceback(tb)
  File "/usr/local/lib/python3.6/dist-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/usr/local/lib/python3.6/dist-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/local/lib/python3.6/dist-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/lib/python3.6/dist-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/viewsets.py", line 116, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/views.py", line 495, in dispatch
    response = self.handle_exception(exc)
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/views.py", line 455, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/views.py", line 492, in dispatch
    response = handler(request, *args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/mixins.py", line 48, in list
    return Response(serializer.data)
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/serializers.py", line 768, in data
    ret = super(ListSerializer, self).data
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/serializers.py", line 262, in data
    self._data = self.to_representation(self.instance)
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/serializers.py", line 686, in to_representation
    self.child.to_representation(item) for item in iterable
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/serializers.py", line 686, in <listcomp>
    self.child.to_representation(item) for item in iterable
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/serializers.py", line 513, in to_representation
    fields = self._readable_fields
  File "/usr/local/lib/python3.6/dist-packages/django/utils/functional.py", line 80, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/serializers.py", line 376, in _readable_fields
    field for field in self.fields.values()
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/serializers.py", line 363, in fields
    for key, value in self.get_fields().items():
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/serializers.py", line 1050, in get_fields
    source, info, model, depth
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/serializers.py", line 1195, in build_field
    return self.build_unknown_field(field_name, model_class)
  File "/usr/local/lib/python3.6/dist-packages/rest_framework/serializers.py", line 1307, in build_unknown_field
    (field_name, model_class.__name__)

Testing

[owocki]

this LGTM, @danlipert ?

[danlipert]

@owocki looks good - thanks @atoulme !

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@be64e9e). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #6346   +/-   ##
=========================================
  Coverage          ?   27.64%           
=========================================
  Files             ?      281           
  Lines             ?    26054           
  Branches          ?     3841           
=========================================
  Hits              ?     7202           
  Misses            ?    18588           
  Partials          ?      264

Impacted Files	Coverage Δ
app/grants/serializers.py	100% <ø> (ø)

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