The one where we create a special case just for iCloud

Summary:
iCloud is pretty weird: sending an email from SMTP/IMAP doesn't save it under the "Sent". This patch introduces a special case for iCloud and a new action to save a message (`save_sent_message`).

I'd like to have your opinion on this, especially about the default vars I added to make things work.

Test Plan: Ran test_sending.py and checked that two files were created.

Reviewers: kav-ya, emfree

Reviewed By: emfree

Projects: #sync-engine

Maniphest Tasks: T478

Differential Revision: https://review.inboxapp.com/D631