Put a separator before the spreadsheets in the dropdown to make it a bit...

... clearer of the separation (lines 122-125 of _assignments_dropdown_menu.html.erb). Code also cleaned up a bit. The `#admin_items` item in _navigation.scss was renamed to `.separator` so it could be reused to separate the assignments/spreadsheets along with the existing separator between admin items/assignments.

The list items also have hover titles too, with the assignments'/spreadsheets' descriptions.