Remove unnecessary apps from INSTALLED_APPS.

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/
Don't explode when a ChangeDescription references an invalid diff.

When a diff is deleted from the database, it doesn't remove the field from any
ChangeDescription objects (since that's buried in a JSONField and there's no
relation between them). This meant that if a published revision of a diff was
deleted, rendering the ChangeDescription would explode.

This change adds some bullet-proofing around the DiffField rendering to avoid
doing anything if the associated diff doesn't exist.

Testing done: Ran unit tests.

Fixes bug 3451.

Reviewed at https://reviews.reviewboard.org/r/6094/
Merge branch 'release-2.0.x'