settings: Move non-generic settings to local settings
base: Change PrettyFilterAdmin to PrettyFilterMixin and let it inherit from object
base: Improve editonly_fields decorator to handle the case when no fieldsets were set
competitions: Fix redundant import and CompetitionOrgRegistratio unicode method
competitions: Create separate VersionAdmin instances for models
competitions: Add make_active Admin action for season
competitions: Initial implementation of SeasonJoinview
settings: Allow extended set of file extensions for attachemnts