Move engine/models to be app-agnostic

Summary:
- Move engine creation to new module (ignition)
- Move `engine_uri` and `db_uri` to config.py. (Note: we should probably make all configuration done in a module instead of processing a flat file.)
- Move autotimestamps to SQLalchemy mixin so it can be used in other modules
- Move other various mixins to their own file
- Fix imports in all the old migrations and tests

Test Plan: Run the tests

Reviewers: emfree

Reviewed By: emfree

CC: spang

Differential Revision: https://review.inboxapp.com/D161