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.
I think what happened was that @muffinresearch added the `hidetext()` while I was working on this last week, but it didn't get merged in for some reason. So it is removed in this branch vs. master, but I don't think I took action to remove it. This is where continually rebasing isn't helpful: a series of commits would show what happened better. On the other hand, I was pretty out of it last week with a cold, maybe I deleted it by mistake. I'm 99% certain I didn't delete it *and* add `text-indent: 101%` as a substitute though, since I have never used a text-indent like that in my life. I can add `hidetext()` back in if that fixes anything which is broken without it.