competitions: Fix obvious bug of calling wrong method of parent object
problems: Add classlevel, school and school_class to each UserSolution
problems: Add note field to UserSolutions
problems: Allow null values for solution in UserSolutions
problems: Use SortedManyToManyField for problems in problemset
views: Use cached property for better performance in Series and Seasons
competitions: Fix typo in the docstring of get_user_ranking method
competitions: Fix get_user_ranking method in SeasonSeriesMixin
competitions: Make competitors property more effective
competitions: Make difference between 0 points and not submitted problem in results table
problems: Save the current profile-specific attributes relevant to the total score in the UserSolution
gitignore: Add .idea to the gitignore
Readme: Update README.md with project logo