Keyboard navigation is not confined to open modals

[owi92]

Issue
When using the keyboard's tab-key to navigate through modals (new-event and new-series were tested), the focus is not confined to that modal and it is possible to tab through the background and rest of the page, while the modal is still open.
In this case it is not possible to exit the modal by pressing 'Esc' or using any other modal-specific key combinations. Eventually the user will cycle back into the modal, but I think this behaviour is generally undesirable.
This might be an issue with tabindex, but I can't confirm yet.

Furthermore, in Firefox, it is not possible to use the tab key to focus the 'previous' and 'next' buttons in the modal's footer as the focus will skip over it - this is, however, working in Chrome (this also appears to be somewhat fixed in Firefox with the new admin-picard ui, though for now tab navigation there is kinda weird as well).
Turns out, this was prevented by a local setting.

To Reproduce

1. Open new-event or new-series modal
2. Navigate through the window using the tab-key
   3. When using Firefox, try reaching the footer
3. Try closing the window with 'Esc' while focus is on background (in any browser)

Environment Information

• OS: MacOS 12.4
• Browser: Firefox 102.0.1, Chrome 103.0.x
• Opencast version with minor release: develop, 12.x

[ziegenberg]

This was partly fixed in #288.