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
Not exactly what I meant something like
```ruby
if player_in?(match)
  #...
end

def player_in?(match)
  current_user == match.challenger || current_user.match.defender
end
```
Obviously needs a better name