Show Users' fullname in review emails

When displaying the name in review emails, show the users' fullname if they have
a first name setup. Otherwise show their username. This change is made in both
html and text emails.

Testing done:
Manual. Tested with email setting on and off.

Reviewed at http://reviews.reviewboard.org/r/4104/
Fixes bug 2965
Fix overzealous file existance caching.

File existence check caching had a flaw where, if a file didn't exist,
we'd cache that it didn't exist, but if it was later pushed and then did
exist, we'd still think it didn't.

We now only cache when it exists. If it doesn't, we don't cache, and
will check again the next time get_file_exists is called.