[JENKINS-74013] Extract inline JavaScript event handler from SubversionSCMSource.java

[yaroslavafenkin]

https://issues.jenkins.io/browse/JENKINS-74013

Testing done

In Multibranch project select Subversion as a branch source. Fill the Project Repository Base and select credentials.

Before the change
After the change

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

[basil]

Thanks!