Refactor advisory conflict management

[dralley]

Pulp should, probably, try to stop merging Errata and do away with as many special cases as possible. Hopefully without rediscovering all the reasons why it was done this way originally. If we can get rid of it entirely, that would be ideal.

At minimum, testing various combinations of bad data needs to be much easier.

One possibility, if we can't completely avoid errata merge, is to make each errata being independent documents, modeled more simply (possibly with one jsonfield), theoretically that could satisfy making the logic simpler while also making it easier to test. (there's some disagreement over whether that it would be simpler FWIW)

[ggainey]

Current state is just All Edge Cases, because advisories in The Real World are always more complicated/uglier than we expect. We def need to refactor to make this more sane - but a lot of discussion/design will be needed here.