Enable alembic migrations and add initial migrations

* add db_migrate script for easy migrations
* add "alembic" to dev-requirements.txt
* add migrations for primary objects, full-text search, and log_events
Enable tests to run using migrations

* tests can now use either migrations or "create_all"
* suppress alembic logging output when running unittests
Pin Jinja2 at v2.6 to avoid infinite recursion issue in v2.7
Enable guessing of fulltext indexer module

* use scheme from settings.SQLALCHEMY_DATABASE_URI to guess
  whether using sqlite.Indexer or mysql.Indexer