Add dashboard link to check run results

[krancour]

Now that there's a prototype in development for a v2-compatible Kashti, this gateway can be enhanced to include links to events/jobs in Kashti in the check run results sent to GitHub.

We can actually make this more general -- not Kashti specific.

If we allow operators to configure this gateway deployment with an optional URL template -- something like https://kashti.brigade2.io/events/{:event_id}/jobs/{:job_name}, then -- then they can fill in an appropriate URL template for any v2 compatible web-based UI / dashboard and this gateway can just make the proper substitutions.

[krancour]

Removing this from the v0.6.0 milestone because there's no urgency to this until some sort of web UI / dashboard exists. One does exist, but it's merely a prototype at this point.