iframe overlays

Currently we sometimes load the content of overlays via ajax. This works well for simple content but gets complicated for more complex overlays. For #535 I need complex overlays with a lot of functionality.

This PR allows to load any existing adhocracy page into an overlay by using iframes instead of ajax. The header, footer and other stuff will be hidden. The benefit is that most pages will work out of the box. You can find more info on how it works in the inline documentation.

There are some (acceptable imho) issues with this:

*   The escape key will only close the overlay if the focus is in the parent frame
*   The iframe will be resized to fit its contents, but only on load
*   overlays are not allowed inside iframes because they might be bigger than the surrounding frame which would look weird
*   the header, footer etc. might be visible for a fraction of a second

I did not yet clean up, e.g. removing legacy ajax code, because I am in a hurry to finish #535. I promise I will clean up asap.