Summary:
- Add FixedPopover, an absolutely positioned popover component to use for swipe snoozing:
- This component needs to be finished, as its current behavior is primarily for the snooze plugin
- Updates popover.cjsx to properly render pointer for thesnooze popover for the `down` direction
- Adds new design assets
- Adds date input to snooze popover
Test Plan: - TODO
Reviewers: evan, bengotow
Reviewed By: bengotow
Differential Revision: https://phab.nylas.com/D2624
