Enhancement: Accessibility improvements

The following accessibility improvements have been made:

- Contextual helpers now have a focus trap, meaning the user can't focus elements in the background while the contextual helper is showing.
- An issue where the current focus would not change when navigating the resource table/tiles with the keyboard has been fixed.
- The contrasts of input borders have been aligned with the official WCAG requirements.
- The space member search in the right sidebar now has a label. Also, the search has been moved from the top of the sharing panel to the members section where all members are listed.
- Leaving dropdown menus via the keyboard navigation now closes them.
- When the sidebar is open and uses 100% of the screen width (mobile view), it is no longer possible to focus elements in the background.
- Focus issues with the right sidebar have been fixed.

https://github.com/owncloud/web/pull/11574
https://github.com/owncloud/web/pull/11591
https://github.com/owncloud/web/pull/11600
https://github.com/owncloud/web/pull/11609
https://github.com/owncloud/web/pull/11659
https://github.com/owncloud/web/issues/10725
https://github.com/owncloud/web/issues/10729
https://github.com/owncloud/web/issues/10724
https://github.com/owncloud/web/issues/10733
https://github.com/owncloud/web/issues/10618
