Implemented some ARIA attributes

This implements a lot of ARIA attributes, but not all possible ones.
There were a few occasions where I wasn't sure how to handle things. It would be great if someone with the knowledge would check this. Although I fear we don't have anyone knowing WAI-ARIA well enough...

What is still missing, but is also not so important (and I don't plan to add to this PR):
* quick search
* link wizard
* media manager (both)
* roles in the template (which I would probably not want to do at all)
* mobile sidebar show/hide in the template