Fix the webapi decorators to properly preserve decorator state.

This makes use of the new fixes in Djblets for webapi decorators to
preserve the response errors and not pollute the original functions.

It also removes decorators that haven't been used in a very long time.
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/