Tidy up the mouseover-highlights for Country Rankings

[teajaymars]

For example, from Delaine:

Can we clarify that the 95 are the questions, and instead of “Survey score” write something like “Share of questions by grade” or something similar. Another thing that Delaine was thinking about (and here come the colors) was to highlight the various letters with different strips

Could also experiment with putting the text within the bars? I've never been very happy with the little popup windows.

[teajaymars]

Closed the wrong issue...