Prevent duplicate committees

[aviupadhyayula]

Clubs sometimes mess up during creating applications, and have multiple committees with the same name.

This PR adds serializer-level checks to prevent duplicate committees tied to the same application. Also adds a DB-level constraint.

The merge_duplicate_committees command retroactively fixes existing applications by merging committees together. This should be run in prod, and then removed from this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 28.57143% with 40 lines in your changes missing coverage. Please review.

Project coverage is 72.00%. Comparing base (7c8281a) to head (b3d7198).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
.../management/commands/merge_duplicate_committees.py	21.56%	40 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #772      +/-   ##
==========================================
+ Coverage   71.98%   72.00%   +0.02%     
==========================================
  Files          32       33       +1     
  Lines        6963     7019      +56     
==========================================
+ Hits         5012     5054      +42     
- Misses       1951     1965      +14     

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