Clicking participant fields to type in them did not cause state.focusedField to change, because no onFocus events were bound to the ParticipantTextFields. Since setState was not called, the focus would appear to change but revert as soon as you touched state.
This diff also renames `onChangeEnabledFields` to `onAdjustEnabledFields` making it more clear that unlike the other handlers, it does not take a new value, it takes a set of changes. I also noticed that we /always/ focus fields when showing them, so I removed the separate focus param from it and made it adjust focus at the composer-view level only.
I also consolidated everywhere that touches `state.focusedField` so that we can keep the `_lastFocusedParticipantField` value in sync with it more easily.
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
