contest page

[kerolloz]

• [x] add functionality to register button
• [x] only registered users can join the contest
• [ ] admin can create password protected contests to restrict who can join
• [ ] add sections for contests according to their type:
• upcoming.. their problems mustn't be seen in neither problems in contest nor problemset pages, user can only register in them
• running now.. user can view the problems in problems in contest page only and join the contest
• ended.. user can view the problems but sure there is no participation or registration