Use GitHub checks to handle CI completion

[Riscky]

Hoff currently has functionality to configure required checks, introduced in #202
This makes use of the context field of the commit status webhook.
This field isn't very well documented, but apparently contains a string that indicates which check was changed (?).
One can set the required checks in the Hoffs configuration, per project (projects.[].checks.mandatory).

GitHub checks can be marked as required, and Hoff could use that.
This way, one could set the required checks in GitHubs interface.
Compared to setting the required checks in the configuration file, to a under-documented value, this is easier to manage.

Specific use case: we want to report different jobs of a CI pipeline to GitHub separately, so all checks are visible in the GitHub interface.
We found keeping the Hoff configuration up to date with which of these jobs are required to quite the hassle.

CC @DanielNoord