Summary:
- FixedPopover now correctly adjusts itself when overflowing outside
window, in all directions
- Updates styles
- Adds specs
- Remove Popover and popover.less, and refactor all code that used it in
favor of the new FixedPopover
Test Plan: Unit tests
Reviewers: drew, evan, bengotow
Reviewed By: bengotow
Differential Revision: https://phab.nylas.com/D2697
