When enabling an extension, we go through a series of steps to ensure

its state is made available both in the database and to the running

application. Part of this involves synchronizing models to the database

and performing evolutions. However, given that this process is a bit

time-consuming and invasive, it's not something we want to run if we

don't have to. Synchronizing and evolving the database isn't a selective

operation -- it applies to all models across all apps -- so we want to

avoid it when possible.
This change makes the process a lot smarter. Now, before attempting to

sync the database, the extension manager will look at the list of apps

provided by the extension and check if any of them provide models. If

so, then the database will be synced. It then looks to see if there are

any unapplied evolutions for the extension before attempting to evolve

the database.
If these operations are not required, they will no longer be performed.
There's still room for future improvement, but they involve much more

careful introspection of the models and the database, and upstream

changes for Django Evolution to allow for selective evolution.

All unit tests pass in Djblets and Review Board.
Ran the rbintegrations test suite against this. It passed, and only

took 12.577s instead of 77.692s (the time required without this

change).