[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.
That's what I would expect as well, but that doesn't appear to be the case. I was using polling and experienced that even though the project was disabled the cron trigger did keep triggering.

I actually noticed this through another bug, namely that [here](https://github.com/jenkinsci/ghprb-plugin/blob/master/src/main/java/org/jenkinsci/plugins/ghprb/GhprbPullRequest.java#L241) it tries to start a build but if that fails because the project is disabled for example it will still set the commit status on the PR to pending. IMO it should only do that when the build was actually started successfully.

There's an open issue on Jenkins core for this too btw, see [JENKINS-10622](https://issues.jenkins-ci.org/browse/JENKINS-10622). It has been open for 3 years. Even if this will be fixed in Jenkins core at some point all older versions will still have this behaviour so I still think we should add the disabled check in this plugin too.