Support Heroku preboot, speed up production deploys.

[toolness]

This adds support for Heroku preboot. If it's enabled, we don't enter maintenance mode at all, resulting in a zero-downtime deploy; otherwise, we behave as normal, entering maintenance mode before doing any migrations.

This basically means that we need to disable preboot if we ever make any migrations that will cause the old version of the code to crash. Once we've successfully done a deploy with Preboot, I'll update our Deployment wiki page with new instructions.

It also prevents tests from being run on the production branch. This is a bit of a bold move but I've provided the rationale in a comment in our circle.yml.

                # This is a bit counter-intuitive, but we've been extremely
                # diligent about only pushing to production once something
                # has been verified to pass CI on master. Because of this,
                # it's wasted effort to run the exact same tests on production
                # when we push to it, particularly since the merge on
                # production is a fast-forward merge and therefore represents
                # the exact same code that's already been tested on master.

Taken together, both changes should make deploys to production much faster without introducing risk.