Add tracking cart functionality

[zoek1]

Description

lets just make a grants.models.CartData object in the DB and store a JSON blob in it and a FK to the user. and do a POST request

Refers/Fixes

#6892

Testing

https://www.loom.com/share/fcafdd80c7124b0891010353c59d6cb7

[codecov]

Codecov Report

Merging #6902 into stable will decrease coverage by 8.59%.
The diff coverage is 62.96%.

@@            Coverage Diff             @@
##           stable    #6902      +/-   ##
==========================================
- Coverage   26.93%   18.33%   -8.60%     
==========================================
  Files         297      285      -12     
  Lines       28373    28308      -65     
  Branches     4153     4148       -5     
==========================================
- Hits         7641     5190    -2451     
- Misses      20458    23101    +2643     
+ Partials      274       17     -257     

Impacted Files	Coverage Δ
app/grants/urls.py	0.00% <ø> (-100.00%)	⬇️
app/grants/views.py	16.30% <27.27%> (-3.07%)	⬇️
app/grants/admin.py	42.93% <85.71%> (+1.65%)	⬆️
app/grants/models.py	41.69% <88.88%> (-5.30%)	⬇️
app/inbox/urls.py	0.00% <0.00%> (-100.00%)	⬇️
app/avatar/urls.py	0.00% <0.00%> (-100.00%)	⬇️
app/kudos/forms.py	0.00% <0.00%> (-100.00%)	⬇️
app/dashboard/abi.py	0.00% <0.00%> (-100.00%)	⬇️
app/avatar/serializers.py	0.00% <0.00%> (-100.00%)	⬇️
app/marketing/country_codes.py	0.00% <0.00%> (-100.00%)	⬇️
... and 90 more

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[owocki]

hmmm this seems to work, but it requires someone on the backend to reassemble the cart from the activity to make sense of it.

seems itd be easier just to have 1 helper function that takes the entire cart data, serializes it, and sends it to the backend to be stored in a blob every time something changes.