Store original message if quote-stripping removes everything.

Summary:
False positives in quote detection can sometimes remove everything, leaving a
blank message.  In this case, we now store the un-stripped message instead.
This is far from a perfect heuristic, but better than nothing.

Test Plan:
Run Message.calculate_sanitized_body on messages both where quote
detection works correctly and where it would strip the whole message. Run a
sync as a sanity check.

Reviewers: mg

Reviewed By: mg

Differential Revision: https://review.inboxapp.com/D463