modify model

Conflicts:
	app/models/problem.rb
Add some methods to model and remove unused fields

Conflicts:
	app/models/problem.rb
Modify view and controller to adapt to model

Conflicts:
	app/controllers/contests/problems_controller.rb
	app/views/contests/problems/index.html.erb
Remove unused file
Fix some bugs
Remove unused comment
Fix submission history
Refactor User model
Add Attendance model