JS/loader: add icon cache prototype

Just playing around, I'm still unsure whether this really helps. The
motivation behind this patch are icons loaded again after not being
displayed for a little while.

Remains disabled unless we know whether and for which scenarios
(cache disabled, HTTPS...) this really helps.
CSS: more space for AVP table headers

This is now possible as font size got reduced
CSS/forms: style them

* login form style should only affect login form
* accessibility: add labels on login page