New edit user UI

References #2422

- Includes the slug and role inputs, but are commented out for now
- User actions (make owner & delete) modal, also hidden
- Slightly re-worked how the Back & Save buttons are hidden & shown on mobile (It's not ideal, but it works cleanly)

If merged like so, hardly anything will change. This PR just lays the pre-styled ground work for the next person to make the MU side of this work. :)
Both the header itself and its div get the `fade-in` class? Seems unnecessary to me