Check for All Mail folder when creating Gmail accounts

Summary:
This is the sync engine half of T495. UI part in progress.

Adds a Gmail-specific config check to GmailAuthHandler.
This checks for an 'All Mail' folder before creating an account.
Throws a ConfigError for the UI to handle if the folder's not found.

Test Plan:
tests/general/test_folders.py - uses a mocked out auth handler.

System test using real credentials coming separately (per spang's suggestion).

Reviewers: emfree

Reviewed By: emfree

Subscribers: kav-ya, mg, spang

Differential Revision: https://review.inboxapp.com/D765