Add dropdown panes to avatars in member listings

[rgilman]

This is at the proof of concept stage ... and it works! When you hover over the avatar in either the main People directory or a group member listing, you get a dropdown that can be filled with whatever useful links and info we like. Here's a screenshot:

This has no styling done whatsoever (e.g. the overlap). Nevertheless, the links are all functional to illustrate what can be done.

[rgilman]

I've uploaded the changed files to bfn.context.org so you can see the results in action.