[generic_imap] Initial generic_imap.

Summary:
[alembic] Patch up the rev history.

[providers] Update the providers table.

[sending] Fix a few more bugs.

Test Plan: auth accounts from all providers, make sure you can send / receive mail from each

Reviewers: khamidou

Subscribers: emfree, spang

Differential Revision: https://review.inboxapp.com/D293