Remove setup_environ from build-media.py

The setup_environ call is deprecated, in favor of setting
DJANGO_SETTINGS_MODULE in the environment. I fixed this in manage.py but missed
it here.

Testing done: Ran build-media.py and saw it work correctly.
Reviewed at https://reviews.reviewboard.org/r/4656/
Turn on USE_I18N by default.

Now that we have some translations to ship, we should have this turned on by
default.
Update for the changes to djblets static media.

This pulls in the stuff required from the djblets static media move. Part of
this means that we'll re-ship the djblets media as part of the Review Board
package, but that's fine.

Testing done:
* Verified that pages like the dashboard and extension list still worked and
  looked correct.
* Ran build-media.py and checked the results.

Reviewed at https://reviews.reviewboard.org/r/4655/
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.