Summary:
- Add an icon to threads that have reminders set
- Show the pending reminder message header in all perspectives, rather than
just the Reminders perspective
- Add the ability to clear the reminder from the message header
- Condense the message header text
Test Plan: manual
Reviewers: evan, juan
Reviewed By: evan, juan
Subscribers: sdw
Differential Revision: https://phab.nylas.com/D4052
