An API for grouping together messages.

This is a long-delayed start for a few different issues:

- fedora-infra/datagrepper#132
- fedora-infra/bodhi#72
- fedora-infra/mobile#31
- fedora-infra/fmn#24
Yeah, it's mentioned in the doc string that it does nothing by default.  Subclasses are supposed to override it.