Move table+column creation into the `create_db.py` script, which is run at setup.

Summary:
Previously we allowed this to run everytime on startup, which broke some alembic revisions by creating new tables before a migration was run.From now on, we should ony be creating tables+columns via SQLalchemy *once* and all subscequent changes done via migration scripts.

Inspried by a chat with Selena Deckelmann. :)

Test Plan: passes all tests.

Reviewers: emfree, spang

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D89