feat: better reject null characters in forms

[jennifer-richards]

Replaces Django's forms.ModelMultipleChoiceField with a subclass that applies the ProhibitNullCharactersValidator to the submitted values before calling the superclass clean() method. This works around an apparent bug in Django that causes it to throw a server error instead of properly marking the form as invalid.

We've mostly seen this abused in the SearchForm, but I've preemptively replaced all our use of the field class.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.91%. Comparing base (187c2c5) to head (201075a).
Report is 216 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7472      +/-   ##
==========================================
- Coverage   88.98%   88.91%   -0.08%     
==========================================
  Files         291      294       +3     
  Lines       40717    41196     +479     
==========================================
+ Hits        36233    36630     +397     
- Misses       4484     4566      +82     

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