Web UI: spinner: give it 10 lines by defaul

I’ve looked into options and this is still the best I could find.

I’d prefer images because of their simplicity, but both GIF and PNG have
quirks: GIFs have poor transparency, which will not work later when I’ll add
the app background, and PNGs also require CSS and JS code.

The CSS-based (http://dabblet.com/gist/7615212) is super smooth in Chrome, and
less so in Safari and Firefox.