Split up files in djblets.util into new apps.
djblets.util has become a dumping grounds of all sorts of random stuff.

In an effort to make things more manageable and future-proof, this

change moves out a lot of our code into new apps.
This change is backwards-compatible. The old files can still be imported

and will import the old classes and functions they provide, but will do

so with deprecation warnings. In Djblets 0.9, these will be removed.
There's also a few PyFlakes fixes I fixed while in the area.

Ran unit tests on Djblets and Review Board. Both passed.
Went through the files after the big unicode_literals change was merged in and made sure all the changes were replicated in the new files (pretty sure I got them all).