Exempt this project from mandatory jobs

In 1f9ba274 a change was made to the mandatory job template which caused pipeline failures.

While annoying in itself that this wasn't caught before merging, it also meant that it was very hard to subsequently revert since reversion MRs would have their pipelines fail.

Solve this in the following manner:

• Exempt this project from mandatory jobs, and
• Include the mandatory job template in this project's CI configuration.

This keeps mandatory jobs running for this project but has the following virtuous side-effects:

• Should mandatory jobs become broken again, this project retains the ability to have reversion MRs opened and merged in a timely fashion.
• MRs in this project which update the mandatory job templates will, in effect, have them tested as part of the pipeline created for the MR branch. A situation like 1f9ba274 would be prevented because the MR branch's pipeline would fail before merging.