Following a design review, adjust the styling of menus and in particular the
groups menu to better match the mocks [1] and create better separation between
different elements.

- Add a 10px margin below section headers
- Add a subtle border at bottom of expanded submenus
- Remove red hover color for menu items
- Make submenu items slightly lighter and darken them on hover
- Reduce font size to match top bar and reduce font weight for submenu items
- Increase submenu toggle caret size slightly (10 => 12px) and darken
  the caret on hover, consistent with the effect when submenu items are
  hovered.
- Avoid hyphenating messages in the footer of group list submenus

[1] https://sketch.cloud/s/D0OMk/a/EbnxQA/play