Support apps hints for Gmail Contacts and Events sync.

Summary:
This revision supports applications proposals to sync contacts and events. The
application that requests the authentication of an account decleare if it is
interested in contacts and events sync (see Application and Proposal in
redwood.git), the hint is recorded in the RDB and is used to start greenlets
as needed.

NOTE: this revision disables event sync for Outlook.com temporarily since we
don't support requesting permission to access them.

Test Plan:
The behavior within this repository should remain the same before and after
this revision. However, some applications of those we've in prod don't require
contacts and events sync; for those applications new accounts shouldn't start
useless greenlets.

Reviewers: khamidou, emfree

Reviewed By: emfree

Subscribers: spang

Differential Revision: https://review.inboxapp.com/D908