Refactor event models

[lukavdplas]

Update the models for events.

Adds EventDescription (i.e. an event description in a source text) -> close #32

As agreed in our last meeting, this removes a lot of the internal structure of events, so we're not so much modelling events as just giving an overview of passages in narrative sources. I removed the modelling of causal links, as well as the idea of breaking up an event into atomic "actions".

So the result is:

• EventDescription describes a passage in a source text, and you can link the source's descriptions of agents/spaces/objects to it. That's basically all the internal structure it has.
• Since the case_study app is used for more high-level abstractions, I did add an Episode model there. Could still be useful for linking passages?
• Also added "key sites" and "key persons" to a case study, which is more a light suggestion. I thought this would be useful in building an interface for a case study. E.g. for the nun rebellion, the Abbey in Poitiers could be a "key site" from which you can explore the data.