[16] Add dropdown menu

[kmonahan]

Addresses #16 . Adds a dropdown menu component. It's not entirely a straight port of the Gesso for Drupal menu so let's say it's "inspired by" the Gesso for Drupal menu.

• Supports three levels of menu items. In theory, it'll support more than three, but I didn't test it beyond that, since if we need additional menu levels, we generally create a side menu at that point.
• Adds keyboard support as detailed in https://www.w3.org/WAI/ARIA/apg/patterns/disclosure/examples/disclosure-navigation/, including support for arrow keys and home/end. Should be pretty close to what we have on Gesso for Drupal now, although WAI has redone their examples and recommendations since we originally built the Drupal version
• Includes responsive display using a <dialog> element for smaller screens
• Intentionally does not include hover support. For accessibility reasons and for better touch support, we've been trying to move away from menus opening automatically on hover. (We could add it in a future PR, of course, but it's not part of the MVP here.)