[JENKINS-25856] Don't trigger PR builds when the project is disabled

Currently when the Jenkins project containing the Github Pull Request builder is disabled this plugin will keep on trying to build Pull Requests. This is IMO unexpected behaviour.

This PR fixes that by explicitly checking if the project is enabled before triggering a build. I've added some test cases but granted they're not as good as they could be since it requires a whole lot of stubbing to get the tests going. I wanted to add an integration test but unfortunately it seems that all existing integration tests are broken.

Fixes https://issues.jenkins-ci.org/browse/JENKINS-25856.
Thanks for your feedback @KostyaSha. I was looking for the best place to put this check, and chose this location because it works for both cron triggers and webhook triggered builds.

Looking at the code structure again I agree that adding the check earlier in the process would be better. What would you think about:

* Putting a check in [GhprbTrigger#run](https://github.com/jenkinsci/ghprb-plugin/blob/master/src/main/java/org/jenkinsci/plugins/ghprb/GhprbTrigger.java#L144), as it already contains a check to abort the trigger of webhooks are used.
* Putting a check in [GhprbRootAction#getRepos(String repo)](https://github.com/jenkinsci/ghprb-plugin/blob/master/src/main/java/org/jenkinsci/plugins/ghprb/GhprbRootAction.java#L85). Filtering disabled projects here would make sense and prevents any communication with the Github API in case the project is disabled.

Please let me know what you think.