Summary:
This diff adds support for database migration to our cloud API. It's partially inspired by Halla's local-sync migration diff (D3809). You can run a migration by calling "node-babel scripts/migrate-db up|down" or by calling "npm script upgrade-db|downgrade-db".
Note that for simplicity reasons we assume that we're only writing migrations for our MySQL database – people developing locally may have to blow up there dbs whenever there's a schema change, though in practice `ALTER TABLE ADD COLUMN`statements work the same on both dbs.
Test Plan: Tested locally. Will run the metadata migration on staging.
Reviewers: evan, spang, halla
Reviewed By: halla
Differential Revision: https://phab.nylas.com/D3840
