Summary:
Keymaps & menus CSON => JSON, remove AtomKeymaps, CommandRegistry use of CSS selectors, use Mousetrap instead
Important Notes:
- The `application:` prefix is reserved for commands which are handled in the application process. Don't use it for other things. You will not receive the events in the window.
- Maintaining dynamic menus seems to come with quite an overhead, because Electron updates the entire menu every time. In the future, we'll need https://github.com/electron/electron/issues/528 to really make things nice. I will be tracking this upstream.
- The format for keyboard shortcuts has changed. `cmd-X` is now `command+shift+x`
Test Plan: Run tests
Reviewers: juan, evan
Reviewed By: evan
Differential Revision: https://phab.nylas.com/D2917
