Implement dropdown menu component

dante.dubon → made their first commit to this issue’s fork.

Gave this a quick manual test. Looks like a good start but isn't quite there yet.

Here's what it looks like now -- the background color is wrong but the menu actually looks partway styled, which is nice:

Clicking on any of those links yields a nasty exception. Not sure if this is a preexisting bug in Mercury, or introduced by this issue:

I added a page under the "Resources" item, but unfortunately, it doesn't show up as a dropdown if I hover over that item. Is there something else I would need to do for that to work? Or is that something we're punting? (Which, by the way, would be totally fine, as we don't expect the initial site template to need nested menus.)

Merged into 1.x with @galactus86's help.