Add GmailAccount

Summary:
Adds new GmailAccount which inherits from ImapAccount to provide storage of Gmail-specific properties, such as access tokens and OAuth details. This also removes prefixes, so account.o_access_token becomes account.access_token in the case of a GmailAccount.

This commit also cleans up some ForeignKey references to use classes+attributes
instead of strings. ie: Lens.id instead of 'lens.id'

Providers now user all lowercase names. e.g. 'gmail'. OAuth is also now keyed
as 'oauth' to avoid bugs in string comparisons.

Both ImapAccount and EASAccount inherit from the base Account table/class.

Test Plan: Tests don't pass yet because the database needs a migration.

Reviewers: mg, spang

Reviewed By: mg, spang

Subscribers: emfree

Differential Revision: https://review.inboxapp.com/D189