Enable HMR for CSS and improve overflow behaviour of bottom panel

[oliver-daniel]

Before	After

(viewport height: 330px)

Also harder to show, but the File queue view is now scrollable in both axes on sufficiently small viewports/panel sizes.

The strangest "hack" of these changes is the addition of:

const Bottom = styled.div`
  display: flex;
  flex: 1;
  + min-height: 0px;
`

But, doing so allows each panel of the editor to handle its own resizing strategy in a way that should be appropriate.