Objective

Add a button to the bottom of the right panel that opens a modal explaining all the keyboard shortcuts available.

Here's an example from Dropbox Paper:

Trello has a similar thing:

Rationale

There is a lot of functionality that can only be accessed via keyboard shortcuts at the moment. Most users aren't going to know about them without a lot of trial-and-error. This is a pretty lightweight and flexible way to make that information available.

Notes

• We should make a standalone Modal component using portals so that we can reuse it for other places where we need modals.
• It'd be nice if the keyboard shortcut modal could also be displayed if the use presses SHIFT-?.