It isn't possible to set the bug supervisor for a source package

At the moment source packages' Bug Supervisor and Security Contacts are inherited from the distro. This means that we have a really coarse-grained model for package maintainers, which leads to all kinds of nastiness W.R.T people having to join the Ubuntu Bugs team just to maintain bugs in one package properly.

We should add Bug Supervisor and Security Contact fields to DistributionSourcePackage so that it's possible to have a finer-grained management of individual packages. However, the distribution Bug Supervisor and Security Contact should still have the necessary permissions on all the distro's packages, and those values would still be inherited where a package has no supervisor / contact of its own.