Ek Add some validations for Match

* Only a challenger or defender can report a winner
* A winner can only be reported once
* Links to report a winner are gone if not permitted
![tumblr_lzpzmkcyil1qlh4hwo1_500](https://cloud.githubusercontent.com/assets/755201/5341792/dcd3adda-7ec6-11e4-8cdb-58f07806daf1.gif)