[generic_provider] Support for generic providers via configuration file.

Summary:
[alembic] Patch up the rev history.

[providers] Update the providers table.

[sending] Fix a few more bugs.

Fixes T227
Fixes T250

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

Reviewers: khamidou, emfree

Reviewed By: emfree

Subscribers: emfree, spang

Maniphest Tasks: T250, T227

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