Fix searching for screenshot/file attachment comment text in the admin UI.

We were referencing incorrect fields in the admin UI for the search
field used to search screenshot and file attachment comments. We now
reference the correct field, restoring search functionality.

Fixes bug #3616
Reviewed at https://reviews.reviewboard.org/r/6496/
Only set a DB index for ReviewRequest.summary.

The previous change I had for adding an index for search added it in
BaseReviewRequestDetails. While we didn't need it for drafts, I figured
it didn't hurt.

However, it exposed a bug in django-evolution, where, presumably due to
the base class, applying the change to ReviewRequest's signature also
affected ReviewRequestDraft's.

I don't have a fix for that yet in django-evolution, so for now, I'm
splitting that field out and turning indexing on only for
ReviewRequest.summary.

Reviewed at https://reviews.reviewboard.org/r/6498/