Add 'No plugins' notice to Appearance configuration page

[janfaracik]

Doing some debugging yesterday with @timja we came across a rather odd issue where on #10245 the Manage Jenkins page would load really slowly, but only when the user was signed in. After a bit of fiddling we roughly identified the issue as something being locked when doing a lookup for Extensions as well as for a user's getTransientActions and getPropertyActions. Would need to do a bit more digging to figure out why.

This is a workaround to the issue, but also improves the user experience a little by not hiding the appearance link when there isn't anything to configure - we instead show a friendly message.

When there are no plugins installed

If anybody has any better ideas for content for this let me know.

Added a little button to the page to get more plugins

Testing done

• Message shows when there are no plugins installed
• Message hides when there are plugins installed

Proposed changelog entries

• Add 'No plugins' notice to Appearance configuration page

Proposed changelog category

/label web-ui,rfe

Proposed upgrade guidelines

N/A

Submitter checklist

• The Jira issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
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Desired reviewers

@jenkinsci/sig-ux @timja

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

Maintainer checklist

• There are at least two (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 is not blocking the change.
• Changelog entries in the pull request 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, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
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[timja]

should probably update the user profile page as well to be consistent.

It currently looks like this:

[janfaracik]

Good spot - updated it + hid the save/apply buttons.

[timja]

@jenkinsci/sig-ux small one if someone could review please

[timja]

@krisstern mind having a quick look? this is blocking the custom header PR