Prevent crashes when unable to get/create a ReviewRequestVisit.

Under some unknown race condition, it's possible that looking up or
creating a ReviewRequestVisit will fail when viewing a review request.
This isn't the worst thing in the world, and is likely a temporary
problem (probably due to two requests for creating the same
ReviewRequestVisit at once).

Instead of crashing, we now log the failure and continue on.

Reviewed at https://reviews.reviewboard.org/r/5959/
Merge branch 'release-1.7.x' into release-2.0.x
Allow for age-based filtering when reindexing for search.

This updates our SearchIndex to tell Haystack which field should be
considered to determine the age of a review request, in order to speed
up filtering.

Now, when running update_index with `--age=<hours>`, only review
requests that have been updated in <hours> will be updated or added to
the index.

Reviewed at https://reviews.reviewboard.org/r/5937/