Make some dropdown CSS more specific

Summary:
Dropdowns have some span rules and such currently. Give them class-based rules instead.

(This allows me to add another <span> to menu items later on without it picking up silly styles.)

Test Plan: In a future diff, added menu items with additional <span>s inside them.

Reviewers: btrahan, chad

Reviewed By: chad

CC: chad, aran

Differential Revision: https://secure.phabricator.com/D7294