Remove unnecessary apps from INSTALLED_APPS.
Review Request #1369 — Created Sept. 1, 2018 and updated
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.) Reviewed at https://reviews.reviewboard.org/r/6085/