PersonEditView validation error can result in HTTP 500

[praseodym]

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/django/core/handlers/base.py", line 124, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/lib/python3.7/contextlib.py", line 74, in inner
    return func(*args, **kwds)
  File "/usr/local/lib/python3.7/site-packages/django/views/generic/base.py", line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/django/views/generic/base.py", line 88, in dispatch
    return handler(request, *args, **kwargs)
  File "/srv/ldb/views.py", line 166, in post
    return self.forms_valid(forms)
  File "/srv/ldb/views.py", line 177, in forms_valid
    obj = form.save(commit=False)
  File "/usr/local/lib/python3.7/site-packages/django/forms/models.py", line 453, in save
    'created' if self.instance._state.adding else 'changed',
ValueError: The CommitteeMembership could not be changed because the data didn't validate.

Datadog trace

[praseodym]

This seems to happen when the deletion checkbox is checked, i.e. this is a duplicate of #113.