Support for multiple Google Calendars + a bunch of fixes

Summary: This diff adds support for syncing and syncback of all the calendars available to an user. I also fixed some "bugs" with the syncback code -- notably, it's now possible to add participants to an event.

Test Plan: Tested manually.

Reviewers: emfree, kav-ya

Reviewed By: kav-ya

Subscribers: spang

Projects: #sync-engine

Maniphest Tasks: T613

Differential Revision: https://review.inboxapp.com/D745