add ui elements
fix username creation for external users; fix degree error message; add missing attribute in UserImporter
skip empty sheets; fix user import
importer: cleanup, refactoring.
importer: handle empty emailadresses better. use clean_fields for additional error checking. check user data correctness in the UserImporter.
importer: fix test
importer: move 20 char limit into settings. minor styling and refactoring.
împorter: localize it
Merge pull request #440 from karyon/importer_overhaul

Importer overhaul
remove uses of is_active, we don't use that
introduce UserProhfile, the new UserProfile. not much working yet.
make a few things work. it doesn't immediately crash anymore
make some more things work. change userprohfile to userprofile again
make most things work
more fixes. make most tests work with the new usermodel
make is_external handle empty email adresses
remove obsolete code
add the PermissionMixin to the UserProfile and introduce the Staff Group. this makes all tests pass.
add some fields do the UserProfile admin
add a handmade userprofile migration