Move theme selection to appearance user page when available

[timja]

Depends on jenkinsci/jenkins#7268 but can be released independently

Testing done

Tested interactively and verified it saves and is gone from the general page

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
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• Ensure you have provided tests - that demonstrates feature works or fixes the issue