Add support for icon on sidebar report action

[jonesbusy]

Inspired by #277 (that was most likely abandoned)

And ensure it support custom symbol

Testing done

Interactive tests with freestyle job with both image from report and symbol

With symbol

With image from report

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

[jonesbusy]

Hi @jenkinsci/htmlpublisher-plugin-developers what do you think of this PR ?