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)
I don't think this is true. You don't have to be an app administrator to be able to create a new organizational unit. For example, in Timetable it would be possible to create new modules without having to be a full-blown app administrator. Being a member of the group that manages one of the organizational units should be enough.