Application-wide feature flags

To go along with this update to the test_track_rails_client Betterment/test_track_rails_client#60, this adds a default "TestTrack" app and an "app_id" identifier type to the seeds file, so that consumer apps all don't have to create their own "app_id" identifier type.

For new apps, the seeds will run as part of rake db:setup, and for existing apps, you will have to run rake db:seed. It is designed to be idempotent, and accept any already existing apps with the name "TestTrack" or existing identifier types with the name "app_id".