Add improvements to the user infobox.

This does a few things to improve the user infobox experience:

1) We now show the infobox for any elements with a "user" class. This
   includes the list of reviewers, the name on each review, and the submitter.

2) There's a 1 second hover time before displaying the infobox, so moving
   the mouse across the page won't suddenly pop up boxes.

3) We have a single infobox element that we share, instead of one per instance
   of the name.

4) We load the infobox data once per name, instead of once per link. It's
   kept in a cache now. This makes infoboxes far less expensive.

5) The infobox is now on the body tag, instead of in the link. This works
   around an annoying Chrome bug where all the text contents end up with
   blue underlines, despite all CSS telling them not to.

Tested the infobox on Firefox and Chrome. It looked correct and appeared in
the right locations after a delay. Also verified through network traffic that
we were only loading one set of infobox content per name (URL, really).