Fix timezone warnings when running search index command.

If the database has time fields which are un-annotated with a time zone, the
index management command will spew a bunch of warnings. If people have this
running via crontab, they'll get an email with all of that.

Fixes issue 2830.
Reviewed at http://reviews.reviewboard.org/r/3742/
Fix file attachment names in changedescs.

When file attachments are added with no captions, the change descriptions would
just say "Added ". This is pretty horrible. This change adds a new property
which will prefer the caption and fall back on the basename.

Testing done:
Added a file without a filename and published. Checked that the change description
text was the file basename. Gave the file a caption, and then removed it.
Checked that the change description text was the caption.
Release Review Board 1.7.2.