make add event modal more accessible

[rlucke]

adding aria-labels, use buttons instead of divs and add some texts for screen readers

[github-actions]

This pull request has conflicts ☹
Please resolve those so we can review the pull request.
Thanks.

[github-actions]

Hi @rlucke
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[github-actions]

Use docker or podman to test this pull request locally.

Run test server using develop.opencast.org as backend:

podman run --rm -it -p 127.0.0.1:3000:3000 ghcr.io/opencast/opencast-admin-interface:pr-745

Specify a different backend like stable.opencast.org:

podman run --rm -it -p 127.0.0.1:3000:3000 -e PROXY_TARGET=https://stable.opencast.org ghcr.io/opencast/opencast-admin-interface:pr-745

It may take a few seconds for the interface to spin up.
It will then be available at http://127.0.0.1:3000.
For more options you can pass on to the proxy, take a look at the README.md.

[github-actions]

This pull request is deployed at test.admin-interface.opencast.org/745/2024-06-26_14-13-04/ .
It might take a few minutes for it to become available.

[Arnei]

Can access more things by keyboard navigation and the screenreader shouts much less nonsense and more actually useful information, I love it.

Overall very minor issues below:

[Arnei]

Can we use role="none" instead, or is that not yet supported well enough?

[github-actions]

This pull request has conflicts ☹
Please resolve those so we can review the pull request.
Thanks.