Move user fixture to core app

--HG--
rename : wger/manager/fixtures/users.json => wger/core/fixtures/users.json
Add dependency to migration
Fix fixture problems during installation
Move language fixture to core app

--HG--
rename : wger/exercises/fixtures/languages.json => wger/core/fixtures/languages.json
Move DaysOfWeek model to core app

--HG--
rename : wger/manager/fixtures/days_of_week.json => wger/core/fixtures/days_of_week.json
extra : amend_source : ee928aa596247c5282620116529a85d7ab3552df