grants: fix team member pk casting

[danlipert]

Description

Fixes casting of user pks when setting team members in grants

Refers/Fixes

bug noticed in pre-deploy tests

Testing

tested locally

[codecov]

Codecov Report

Merging #5592 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5592      +/-   ##
==========================================
+ Coverage   30.26%   30.26%   +<.01%     
==========================================
  Files         247      247              
  Lines       21143    21142       -1     
  Branches     3060     3060              
==========================================
  Hits         6398     6398              
+ Misses      14469    14465       -4     
- Partials      276      279       +3

Impacted Files	Coverage Δ
app/grants/views.py	15.47% <0%> (+0.03%)	⬆️
app/dashboard/views.py	12.88% <0%> (ø)	⬆️
...rketing/management/commands/no_applicants_email.py	0% <0%> (ø)	⬆️

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