Probably should create a mixin for this: when a model is used as a
task queue for a cron, the cron should automatically be triggered on
creation. Requiring an explicit trigger after a creation is error
prone and increase the risks that some of the triggers will be
forgotten/missed.
Requires parsing the commit messages as github plain doesn't collate
the info from there, but also the descriptions: apparently github only
adds those to the references if the PR targets the default
branch. That's not a limitation we want.
Meaning at the end of the day, the only thing we're calling graphql
for is explicit manual linking, which can't be tested without
extending the github API (and then would only be testable on DC), and
which I'm not sure anyone bothers with...
Limitations
-----------
- Links are retrieved on staging (so if one is added later it won't be
taken in account).
- Only repository-local links are handled, not cross-repository.
Fixes#777
