StashBuildTrigger: Only allow jobs derived from AbstractProject

[proski]

This hides Stash Pull Request Builder from the build trigger list in the pipeline configuration. Pipelines are currently unsupported.

Once the pipelines are supported, isApplicable will be adjusted to include them.

[jakub-bochenski]

@jimklimov I would like to merge this ASAP to reduce user confusion

[Chocrates]

@proski Why do you say Pipelines are unsupported? As of version 1.7 of this plugin, the plugin worked fine with pipelines. I will test master against my set up (after rolling this change back) to confirm if it still works shortly.

[proski]

@proski Why do you say Pipelines are unsupported? As of version 1.7 of this plugin, the plugin worked fine with pipelines. I will test master against my set up (after rolling this change back) to confirm if it still works shortly.

There was no version 1.7. The last released versions were 1.6.0, 1.7.0, 1.7.1, 1.8:
https://github.com/jenkinsci/stash-pullrequest-builder-plugin/releases

Maybe you used some unofficial version? Or maybe you mean pipeline views? Or maybe you mean that pipeline jobs can coexist with free-style projects that use this plugin?

Knowing the amount of changes in #69, I doubt that version 1.7.0 or 1.7.1 would do anything useful in a pipeline project.