Make drop-downs more accessible on small screens

- Restore default :focus highlight
- Figure out the right voodoo for Drop to not disappear when out of
bounds, move as necessary, and stay pinned to allow scrolling around.
- Modify max size and padding of drop-down and increase drop shadow
- Add .close icon, markup pattern, CSS (in Drop-downs, at least), and
new Brenda (The Closer :-P) script to manage them
- An accessibility change-up: the main menu doesn't init Prospectus to
keep all links tubbable (for now)

Related to issue #11