Create an admin monitor for warning about duplicate events from GitHub

[gbhat618]

Demo draft; admin monitor works

Desc	Screenshot
Admin monitor works
Compensatory Periodic cleanup works (in addition to on cleanup upon new event)

Pending items

• Type of Github events to watch for (currently I don't understand most of the GHEvent types; currently only put PUSH for making the end-to-end GH webhook working
• The cleanup of old events need to be checked again

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

[gbhat618]

PR created in the OSS repo jenkinsci#388