Move theme selection to appearance user page when available

[timja]

Depends on https://github.com/jenkinsci/jenkins/pull/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
• [ ] Please describe what you did
• [ ] Link to relevant issues in GitHub or Jira
• [ ] Link to relevant pull requests, esp. upstream and downstream changes
• [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue