Cockpit removal, Live ISO fixes

[lslezak]

Problem

• Fix Agama to work without Cockpit

Solution

This pull request contains several improvements:

• Use actions/checkout@v4 in the GitHub Actions, the v3 is deprecated because it uses old Node.js version 16 which is out of support, v4 uses Node.js version 20. (Later we should probably do this change in YaST repos as well...)
• Do not hardcode the target OBS project (systemsmanagement:Agama:Staging) in the GitHub actions code, make it configurable using GitHub Action variable OBS_PROJECT. This allows automatic submitting from your Git fork to your OBS branch and build your own Live ISO easily. (I'll document it in a separate PR, this PR is already too long and I need to do it from scratch to not forget some step I did. 😄 )
• The submit GitHub Actions can be now started manually from the GitHub web UI. Again, this is useful in forks.
• Removed manifest.json file, not needed anymore
• Removed some Cockpit configuration files from the Live ISO
• Deleted duplicated agama.kiwi and config.sh files (I made a mistake when adding to Git). The ALP removal was re-applied to the correct files.
• Added user.js.template file to set the Firefox home page correctly
• Package cockpit-agama-playwright renamed to agama-playwright
• Added some _service files from OBS to track them in Git as well
• Do not load Cockpit's manifests.js file in index.html, not needed anymore
• Updated documentation about languages.json

Testing

• Tested manually
• The Live ISO starts Agama and it can be used both locally and remotely (see https://build.opensuse.org/package/show/home:lslezak:branches:systemsmanagement:Agama:Staging/agama-live)

[lslezak]

Note for myself: Use squash merge when merging this to have a cleaner history.

[imobachgs]

LGTM. Great work 😉