Summary:
We now have a `MessageItemContainer` class that handles the logic of
deciding what kind of message to show. We introduce a new `PendingMessage`
(which is just a sublcass of `MessageItem`) that has the spinner and
stuff.
Also tests
Test Plan: edgehill --test
Reviewers: bengotow
Reviewed By: bengotow
Differential Revision: https://phab.nylas.com/D1833
