Modules only need to be in INSTALLED_APPS if they provide models,

templates, template tags, or other such things. We have a few entries

listed that do not need to be there.
One of these, djblets.webapi, introduced problems when we added models

to reviewboard.webapi. App labels must be unique in INSTALLED_APPS,

due to several assumptions in Django, which was the source of this

problem. (Note that we still violate this all over the place, but it

doesn't seem to matter too much right now for the others.)

Tested that the dashboard, review UIs, API, and SSH still work.
All unit tests pass.