Move best submissions link to statistics page

[kiritofeng]

Currently clicking on the problem number on the contest ranking table takes you to the best submissions for the given problem in contest.

#1758 has mentioned that this leaks the problem codes, in addition, this is counterintuitive. The proposed solution is to:

• [ ] move the current link to the statistics page (these links shouldn't be visible in-contest)
• [ ] replace them with links that lead to the problem that are only active when the user has finished the contest, or is a setter / staff / etc.