#23 - Global admins should not be added as group members
Application admins can always manage organisational units
Don’t make the current user an event organiser if he’s a global admin
Organisational units - Subscriptions
Refactored OrgUnit / Serie subscription

* Moved the adding/removing to/from the calendar to the UsersDAO
* Added lots of tests
* Cleaned up some code
Allow you to specify a context when subscribing to a serie

This allows the UI to specify an organisational unit as context when
subscribing to a serie. We need the UI to tell us what the context is
as series can be borrowed under multiple organisational units and the
backend has no way of knowing which one is the relevant one.