Thanks Tim Graham for the review.

Thanks Tim Graham for polishing the patch, updating the tests, and
writing documentation. Thanks Carl Meyer for shepherding the DEP.

Thanks to Tim Graham for the review.

Also renamed app_cache to apps and "app cache" to "app registry".

Deprecated AppCache.app_cache_ready() in favor of Apps.ready().

It's now easier to achieve the same effect with modify_settings or
override_settings.

Reverted 4a56a93c.

Removed some exception masking in the comments app that was harmful and
couldn't be preserved easily.

Exposing the level name (e.g. "info") makes it possible to prepend
something to the class name. For example, Twitter Bootstrap has
an alert-info class. This class can now be added to the message
using `class="alert-{{ message.level_tag }}".
Because the level_tag was on the end of the `tags` property, it
could not be used in this fashion when extra_tags were given.

Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.

Thanks apollo13, carljm, shaib, akaariai, charettes, and dstufft for reviews.

Refs #20680.