Optional build triggered comment, make "Build finished" portion of comment user-configurable

In my particular instance of Jenkins and GitLab we are attempting to reduce email spam, so I wanted to make the initial "Build triggered" comment optional (i.e. just tell me the results!).

I also made the "Build finished" part user-configurable so you can start the comment with +1 or -1 (or :+1: / :-1:).  This way GitLab can pick up the build result as a vote if desired.