Enhancement: Accessibility improvements

Across the board, we have implemented improvements in regards of accessibility for the web UI.

https://github.com/owncloud/web/issues/5387
https://github.com/owncloud/web/issues/5398
https://github.com/owncloud/web/pull/11087
https://github.com/owncloud/web/pull/11096
https://github.com/owncloud/web/pull/11098
