Adding header to top of citation pop-up list in mobile view

Per https://trello.com/c/xCo6uLAZ/9-5-references-dropdown-from-toolbar-styling

Also adding down-arrow indicator next to icon in handler (per Kaity)

Change-Id: I637891232f40ad217b4d3bd62bbecc502bf4cefb
Dependency: Id54100c55b05bdae5858ca72e4fd4418ceff0454