In the security warnings popup, show recommended actions

[daniel-beck]

This adds information about the (un)availability of fixes to the security warning popup.

The warnings are changed into an inner class to have a reference to the related update site to check whether an update fixing the issue is being offered. We're not bothering with understanding the semantics of plugin updates offered across different update sites here, which might be the correct thing to do or not, unsure. I expect this is very unusual, so we have very little data to reason about the optimal behavior, and it's probably not worth the time either.

Screenshot:

Before

After

Proposed changelog entries

• Show recommended actions (e.g., to update affected plugins) in security warnings popup.

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.
• New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content-Security-Policy directives (see documentation on jenkins.io).
• 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).

[Wadeck]

Thanks for update on it. I imagine the recommendation to uninstall the plugin without a fix is "sufficient" to be in written format like this 👍

[NotMyFault]

This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!