allow half-stars in aggregate ratings (bug 971205)

This involved rewriting the SVG and applying it as a clipping path to a span (and a span:after) on the page using CSS. One span is the grey background, the other span is the blue background, both are overlaid on each other with the same clip path and depending on the number of stars to be show, the length of the blue one is changed.

Tested in both regular and small sizes (small only shows on mobile) in various views. The number of stars gets cached locally, so half-stars may not show up until the app is rated again.
is this how other sites do it? I'm not sure we should be rounding up. that would mean that an app could get a full 5 stars even if all the ratings aren't positive. (I know this is impossible on Amazon since they use a true percentage of the stars.)