Summary:
Participants now collapse Gmail style in the composer field.
New, more declarative system for how we deal with "focusedFields" on the
composer.
Extracted a `CollapsedParticipants` and `ExpandedParticipants` component.
Test Plan: TODO
Reviewers: dillon, bengotow
Reviewed By: bengotow
Differential Revision: https://phab.nylas.com/D2013
