Add the ability to ignore push events from certain users

[negasora]

This adds the ability to specify (per-job) usernames that should be ignored when determining whether or not to trigger a git poll from a GitHub webhook event.

This addresses JENKINS-48139 and JENKINS-24208

There have been a handful of PRs that have tried to fix this or similar issues (#58, #182, #83). They have either been closed or abandoned (afaict).

Testing done

Added testcases in GitHubPushTriggerTest.java and DefaultPushGHEventListenerTest.java

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