Fix so users with apostrophes won't cause 500 errors.

It's possible to get a user with an apostrophe due to active directory. This
change allows the regex to recognize these users.

Testing Done:
I created a user with an apostrophe in the name and went through the steps to
reproduce the problem (going to users page, as well as trying to assign the
apostrophe user to a review). 500 errors were no longer thrown after my
changes.

Bugs closed: 2890

Reviewed at https://reviews.reviewboard.org/r/4606/
Allow plus signs in usernames; update docs.

Testing Done:
All unit tests pass except perforce-related ones, which were skipped, since I
don't have it installed.

Reviewed at https://reviews.reviewboard.org/r/5036/
Fix an exception in the API when accessing file attachments without review UIs.

The thumbnail field in FileAttachment was accessing the review_ui without first
checking it for None-ness. Trivial fix.

Fixes bug 3145.

Reviewed at https://reviews.reviewboard.org/r/5032/