Revert "Avoid circular dependencies"

This reverts commit 6bcc93985defcda0676524b9bb9572c659d27a68.
Revert "Tasks are not assigned to a contest in the database"

This reverts commit 55b2debed307b9dc63a8334dfbba32cc90a460e9.

Conflicts:
	cms/db/task.py
Revert "Roba a meta"

This reverts commit 0e501f5ef4f922cbfe2abf7710c0bde64607b54a.

Conflicts:
	cms/db/contest.py
	cms/server/ContestWebServer.py
UserContes -> Participation