[JENKINS-60866] Extract inline JS from update center

[daniel-beck]

See JENKINS-60866.

Needs Behaviour.applySubtree() because the checkbox is part of the section that gets reloaded.

Reduced diff: https://github.com/jenkinsci/jenkins/pull/6870/files?w=1

Proposed changelog entries

Too minor

Proposed upgrade guidelines

N/A

Submitter checklist

• [ ] (If applicable) Jira issue is well described
• [ ] Changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developer, depending on the change) and are in the imperative mood. Examples
  • Fill-in the Proposed changelog entries section only if there are breaking changes or other changes which may require extra steps from users during the upgrade
• [ ] Appropriate autotests or explanation to why this change has no tests
• [ ] New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadoc, as appropriate.
• [ ] New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO") if applicable.
• [ ] For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@mention

Maintainer checklist

Before the changes are marked as ready-for-merge:

• [ ] There are at least 2 approvals for the pull request and no outstanding requests for change
• [ ] Conversations in the pull request are over OR it is explicit that a reviewer does not block the change
• [ ] Changelog entries in the PR title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood
• [ ] Proper changelog labels are set so that the changelog can be generated automatically
• [ ] If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (example)
• [ ] If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[github-actions[bot]]

Please take a moment and address the merge conflicts of your pull request. Thanks!

[basil]

Manually verified as of commit 93f69b292e by updating a plugin, clicking the "Restart Jenkins when installation is complete and no jobs are running" button, observing that the checkbox remained checked after refreshing the page but before the restart, and observing that Jenkins restarted itself after plugin installation was complete