Summary:
- Factor out focusing logic from Clipboard to FocusLayer.
- Generalize FocusLayer to support adding a temporary layer while a modal is open.
- Stop Mousetrap shortcuts while a modal is open.
- Refactor how Mousetrap's custom stopCallback is implemented to avoid
needing to bundle knockout for mousetrap.
Test Plan: Added a test that Enter in a UserManager doesn't open a cell editor from underneath the modal.
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D2626
