Align shutdown banner, starvation and add a sidepanel to the plugin uninstallation view

[NotMyFault]

The shutdown banner does now have less sharp edges and aligns with other top banners, e.g. update center warnings:

Additionally, I aligned the executor starvation icon with other build widget icons like cancelling builds or the offline widget:

The uninstallation view does now have a sidepanel as well.

Proposed changelog entries

• Align shutdown banner with other top banners, align executor starvation icon with other build widget icons, and add a side panel to the plugin uninstallation view.

Proposed upgrade guidelines

N/A

Submitter checklist

• 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

Desired reviewers

@jenkinsci/sig-ux

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).

[timja]

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!