Syncback "spam" and "trash" tags

Summary:
Kavya, Eben,

This is a basic implemention of the "mark as spam" and "trash" actions. Could you tell me what you think of it?

Test Plan: Triggered the actions using the API on gmail and fastmail.

Reviewers: kav-ya, emfree

Reviewed By: emfree

Projects: #sync-engine

Differential Revision: https://review.inboxapp.com/D449