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/
Fix the Watched Review Request resource with local sites, and add tests.

The GET handler for the Watched Review Request resource didn't accept
local IDs for review requests, causing it to be pretty useless there.
Now it takes the proper IDs.

Unit tests were added for this, and for all other missing cases.