Restyle the Admin UI dashboard for desktop/mobile and simplify operations.

Review Request #1774 — Created Nov. 4, 2019 and updated — Latest diff uploaded

Review Board
The old admin UI dashboard was still themed much like the Review Board
1.6 and 1.7 releases, and was very much showing its age. It wasn't
mobile-friendly, and had a sort of half-working support for
adding/removing/reordering widgets, which probably nobody used.

This change restyles the admin UI dashboard to fit in better with the
rest of the modern Review Board UI. It's cleaner, puts content first,
and works on both desktop and mobile.

It also temporarily removes the ability to reorder and to add/remove
widgets. These abilities are planned to come back, but with a different
implementation. For now, the "X" for deleting a widget still exists, but
won't work correctly.

Widgets themselves have a modern component-based set of CSS classes.
They're responsible for any of the styles they need within their content
area, rather than relying on some very situation-specific defaults that
were previously defined.

Upcoming changes will improve the layout of widgets, rewrite the
JavaScript backing to fix a lot of interaction quirks, add a better
structure for defining widgets, improve the representation of
information in existing widgets, and will either bring back the ability
to add/remove/reorder widgets or remove those capabilities entirely.

Testing Done:
Tested that the widgets still work in mobile/desktop mode, and transition
between the modes correctly without rendering glitches.

Reviewed at