Accessibility fixes

Some first low hanging fruit on accessibility.

I put 2 bigger tasks in separate issues:
* Need alt text on Rename icon https://github.com/owncloud/core/issues/12067
* Rows in sidebar & content not highlighted on keyboard navigation https://github.com/owncloud/core/issues/12068

Please review @owncloud/designers @jerrac @schoeppi @lotophage