Merge pull request #14 from rbreu/master

Menue zum Aufklappen
Menue: z-index
registrations: model and import
import registrations from Wusel-style XLS file
import registrations: list group names
registration admin: filter by 'status' added
registration action 'assign groups' added
query registrations
query 'changed assigned groups' added
'Print group list' added
'print students' (matrikel, name, subject, group)
old 'print students' is 'print exercises'
StaticData: validation of json strings for subject and group translation
Import registrations: update group_translation added
New assigned groups: also list students without previously assigned group
Query assigned groups: display number of new assigned groups
Query new assigned groups: list students with and without old assigned group
	separately.
export students to CSV file - preliminary
query assigned groups: exclude students without matrikel,
	order by matrikel
export students completed
'print exercise sheet form' added