Finish Modeling User Accounts

This finishes up modeling the user accounts in Warehouse. Major highlights are:

* GPG Keys are given a place to be stored
* ``conversion/user_accounts.sql`` was finished to enable turning the PyPI ``users`` table into a Warehouse ``accounts_user``, ``accounts_email``, and ``accounts_gpgkey`` tables.
* Updates to validation to push more things into the database and be stricter in some areas.