Fix comments in the diff viewer.

When I refactored everything to let the diff viewer change revisions without
reloads, I broke comments on the diff page pretty hard. This change fixes it
and cleans things up a bit.

Testing done:
Verified that I could see comment flags (and the associated comment
tooltips/dialogs) on the diff page for various revisions and interdiffs.

Reviewed at https://reviews.reviewboard.org/r/4677/