[GitHub App] Optional allow the PR check to fail when prebuild fails

[svenefftinge]

When a prebuild fails some users might want to treat this as a blocker to merge a PR, because it might break the dev environment.

Proposal

Add an option to the GitHub App configuration (currently in .gitpod.yml, should move to dashboard/project btw.) which makes PR checks block when the prebuild failed.

[svenefftinge]

Oops, my assumption that this is easy to do doesn't hold anymore as this PR did remove the GH commit status code.