Add a mechanism to skip triggering builds.

[randymarsh77]

This PR addresses #17

Draft because: needs testing.

Skip when:

• The PR is a draft
• The PR has a 'skip-build' label

Always build when:

• The PR has a 'build-me' label

[bgrainger]

Unfortunately, createNewCommit (which creates the lprb- branches) is called before startBuilds, so this will create a bunch of unnecessary lprb- branches in all the repos (and never* clean them up, because that only happens when a Jenkins build notification is received, which won't occur for Draft PRs).

* Maybe not never; I think if the PR was converted to non-draft, those branches would get moved, then built, then cleaned up? No, that's incorrect. The branches aren't reused; a unique name is created every time.

[randymarsh77]

I've updated the code in accordance with the discussion on the related issue. I've also moved the check earlier in the process to avoid creating commits.

For simplicity:

• I didn't add a trigger on label change; A PR must be created/opened with the label. If the label is removed, then the rebuild this comment can be used to trigger a build.
• The logic only checks top-level PRs; It was hard to reason about what return types to use and how to refactor the recursive call into promise chaining... maybe if we convert all the code to async/await it would be easier/safer to make this feature "more robust".

[bgrainger]

buildPullRequest returns a ReplaySubject; buildPullRequestIfNeeded returns a Promise; can those be used interchangeably?

[randymarsh77]

It looked like the places I changed to call buildPullRequestIfNeeded weren't using the return value, so I think that shouldn't matter.

[bgrainger]

Should this Promise be returned? I can't see anything using it. (Or should something "listen" to it?)

[bgrainger]

Suggested change

	});
	})
	.then(x => null, err => { ... log.error ... });

Should we add a catch-all error handler here to log any errors? Just concerned that failure might be silently dropped on the floor.