This introduces a new Evolver class, which is responsible for managing

the evolution process. Previously, the logic for managing this existed

only in the evolve management command, which was hard to extend and

use in other projects.
Evolver uses a task-based system, where evolution-related tasks are

first queued up and then, when the caller is ready, either executed or

used for information gathering purposes (such as evolution hinting).

Callers can queue up general project-wide evolutions/purges or can queue

up the evolutions or purges of individual apps. They can also queue up

specialized tasks, if they have a need to create them.
Going forward, additional tasks may be created to help with creating

model baselines, call into Django's migrations, or perform other

operations we may need.
The evolve command itself has been rewritten to wrap the Evolver.

This massively simplifies the code of the command, taking the logic from

years of spaghetti code to something straightforward and maintainable.

Unit tests pass on all databases.
Manually tested the various evolution operations against several

generations of Review Board schema evolutions. There may be

regressions still, but functionality appears to work as expected.