The `sender` attribute in signal handlers is special, as it will
always be sent, and is thus not part of the `kwargs`. However it
still makes sense to allow access to the sender in the `filter_events()`
predicates

Part of a series of PRs that will lead to the analysis module.

Add a denormalisation step to the structural data models (case, workitem,
and document) that will allow faster queries across case structures down
the road.