Enhancement: Improve accessibility for the files sidebar

We've did several improvements to enhance the accessibility on the files sidebar:
- Transformed the file name to a h2 element
- Transformed the "Open folder"-action to a link instead of a button
- Transformed the favorite-star to a button-element
- Adjusted aria-label of the favorite-star to describe what it does instead of its current state
- Added a more descriptive close button label
- Clicking outside of the sidebar now closes it
- Removed the aria-label on the action buttons as they already include proper labels
- Added a hint for screen readers if an action opens a new window/tab
- Make sidebar header sticky

https://github.com/owncloud/web/pull/5000
https://github.com/owncloud/web/pull/5266
