I forgot that many segfaults are *not* spurious failures.

Since web_url doesn't come for free with the webhook payload, go ahead
and just fetch the API Job object to get it. Simplifies some
argument-passing as a bonus.

The database now has a constraint on (job_id, type), and we can just
ignore conflicts.

This fixes a bug where only jobs on the GHC project were being processed!

This causes the gitlab-bot to exit more gracefully when run against a
job with no artifacts.

Still not sure what happens when it runs against a job that has the
*wrong* artifacts, though. :)