Add .active class for topbar dropdown items

Previously, the `.active` class was being overriden by the `.dropdown`
selectors in the `.top-bar-section`, thus not allowing active dropdown
items to be highlighted.

This commit adds another pseudo-selector to ensure the `.active`
dropdown items get the correct highlight colors. It also works in
dropdowns within dropdowns within dropdowns.
Merge pull request #5622 from gonzedge/add-active-class-to-topbar-dropdown

Add .active class for topbar dropdown items