Generalize the validator for regex form fields, and add a test for an invalid regex.

This moves DefaultReviewerForm.clean_file_regex out into a reusable validator
that can be added to any field. In addition, I've added a test for attempting
to create a default reviewer with an invalid regular expression.

I did this incidentally while banging my head against failures caused by the
changes to PUT in the test client, but it's a good change nonetheless.

Testing done: Ran unit tests
Revewed at https://reviews.reviewboard.org/r/4616/