Add tokens column to Tasks.
Able to set tokens for tasks.
Implement ended and participated_by scopes for Contest.
Implement Task#submissions_left_for.
Show number of tokens left for each task on problem page.
Implement Task#allowed_to_submit?.
Makes use of #allowed_to_submit? on problem page.
Ensure that the user is allowed to submit before accepting the submission.
Fix a bug to prevent task tokens from being updated.
Merge branch 'tokens'