Organisational units, series, events and groups

Pretty massive PR including:

Organisational units:
 * CRUD
 * Getting / Adding / Deleting series from an organisational unit
 * Getting the organisational unit tree

Series:
 * CRUD
 * Adding / Deleting events to/from series

Events:
 * CRUD


It's pretty rought, but I'll go do some self-review (which hopefully gets in before @nicolaasmatthijs gets on the plane)
Maybe we can make this the `/api` endpoint for the event? At least that will be there for both applications