Accounts: Add a custom User class

This can be separated out from the deletion of profiles. Here we had a User model, mostly similar to evernote's, and use it (through `AUTH_USER_MODEL`). This is NOT gtm and will currently only work on postgres. See the migration for more info.

Builds on https://github.com/evernote/pootle/commit/8228637df5d5e2eee216446da6e7d7503d836a41.