Fix crasher when Gmail 'Important' folder disabled in IMAP.

Summary:
We solve this by creating folder rows with a NULL 'name' when we have
the special Gmail label on messages but the corresponding IMAP folder is
disabled. If the folder is later re-enabled, we reconcile the existing
Folder row with the detected folder name, so messages will be properly
synced.

Note that adding `nullable=True` is merely making an existing default
explicit.

Test Plan: Manually disable Important on test account, sync messages, re-enable and verify resolution.

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D232