Refactor and consolidate different keyboard logic in JavaScript code

[ellisonbg]

We currently have keyboard logic in a couple of different places:

• base/js/utils.js: old keycodes, some testing utils.
• notebook/js/keyboardmanager.js: new keycodes, ShortcutManager, KeyboardManager.

This should be refactored into:

• base/js/keyboard.js: new keycodes, ShortcutManager, utilities for testing.
• notebook/js/keyboardmanager.js: KeyboardManager`.

When this is done, we should also refactor the entire codebase to use the new keycodes and logic in keyboard.js. This should be done after the widget PR is merged: #4374