Speed up search indexing.

Search indexing was painfully slow, since every single ReviewRequest
being indexed ended up adding one query for the submitter and one to get
a list of all FileDiffs.

The FileDiffs query in particular resulted in quite a lot of unnecessary
time spent.

By taking advantage of select_related(), prefetch_related(), and a
prepare_file() statement, we can reduce this down to only 2 additional
queries per batch of review requests (as defined by Haystack, defaulting
to 1 large batch).

Reviewed at https://reviews.reviewboard.org/r/5935/