base: Add South introspection rule for ContentyTypeRestrictedFielField
problems: Add migration for ContentTypeRestrictedFileField change in UserSolution
problems: Make only problems from organizers competition visible
base: Improve RestrictedCompetitionAdminMixin
admin: Restrict existing admins with respect to competition fields
events: Add competition reference field to model
events: Add restriction for the competition reference
base: Remove carousel from the site
templates: Remove unnecessary subheadings
base.util: Change with_author decorator with respect to localization
competitions.models: Localize hardcoded strings