Detect misconfigured Gmail accounts.

Summary:
We require the 'all mail' folder to be enabled via Gmail IMAP. This
commit hard-aborts syncs for accounts that are misconfigured, and
directs the user to fix their account settings.

Test Plan: runtests, manual testing with setting disabled

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D230