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.


This is *NOT* ready for an in-depth review. I'm mainly looking for feedback on the `context` mechanisme in CalendarSeries and whether this is the way to go really. I also couldn't figure out a way to join the context orgunit row from the join table, hence the silly `getOrgUnitsByIds` :(
We can just do `DB.OrgUnit.findAll({'where': {'id': ids}})...` like elsewhere