"Restore" ESLint support

[JulianKniephoff]

I removed our leftover config in #700 with a promise to bring ESLint back. Well this is it, for now!

This introduces an ESLint config file in the new flat file format, which will be required for ESLint >= 9. It is already supported in version 8, though, which we are still stuck on because of the (unmaintained) config we are/were using. I briefly tried to port this project over to @opencast/eslint-config-ts-react, but the code said "no," basically. ;P

This has a few unfortunate consequences. See #713. I plan to address these in due time, though.

Note that this does not introduce automatic/live checking of the code during development. I only added a call to the linter to the build command, in turn making CI fail when there are warnings. I created #712 to track the live thing if we want that.

[github-actions[bot]]

This pull request is deployed at test.admin-interface.opencast.org/714/2024-06-25_16-18-46/ . It might take a few minutes for it to become available.

[github-actions[bot]]

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-714

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-714

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[bot]]

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