task management

[ArcCha]

As for now task contains description, name, shorthand for name and chosen test environment. To avoid problems with tasks changing over time (for example when someone discovers that the test suite is not good enough and they add new tests) we introduce a concept of TaskEdition. TaskEdition has additional required information like start date, due date, possible score etc. Each task can have multiple editions in multiple contests. When task is going to be added to contest it is actually copied so any possible future changes are applied to a copy, so they do not impact on older editions of same task. All of this needs to be implemented.