Fix prepare-dev.py to not link djblets static media

Now that djblets is fully pipeline-ized, we don't need to create the static
media symlink. This was extra broken, because djblets/media doesn't exist
anymore, so new users trying to use prepare-dev.py would just get an error.

Reviewed at https://reviews.reviewboard.org/r/4812/
Document extension API resources.

Extensions could add their own resources to the WebAPI, but this was never
documented. I've added a sample of this that looks a lot like Review Bot's
review resource, but simplified quite a bit to eliminate a lot of the actual
guts, leaving only a shell of a resource.

Testing done: Generated HTML docs.
Reviewed at https://reviews.reviewboard.org/r/4818/
Switch to using allowed_mimetypes in webapi resources.

The allowed_item_mimetypes and allowed_list_mimetypes were deprecated in
Djblets 0.6, but a few of our resources still use them. They'll be nuked
from Djblets soon, so I've updated our code here to no longer use them.