Fix registration when school details are not required
Registering as guest does not require school details
List of allowed grades for registration.
District model and administration interface
User may belong to a district.

Districts are not exported together with a contest. District name is exported as a reference to the district. Similarly when importing.
TWS: beginnings and login
Result list and contestant impersonation
TWS: translation
Utility to import districts from file
Merge branch 'registration' into lmio2015

Conflicts:
	cms/db/contest.py
	cms/server/po/lt.po
	cms/server/po/messages.pot
	cms/server/templates/admin/contest.html
TWS: Log impersonation actions
Merge branch 'teacher_web_server' into lmio2015

Conflicts:
	cms/db/__init__.py
	cms/db/user.py
	cms/server/AdminWebServer.py
	cms/server/po/lt.po
	cms/server/po/messages.pot
	cms/server/templates/admin/add_user.html
	cms/server/templates/admin/user.html
District choice in student registration
Do not allow contestants to post blank questions