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.

Ran unit tests