A User can report the winner of a Match

* At the moment anyone can report the winner
* Link to report match on dashboard and match show page
* Create Edit and Update controller action for match
* Change winner to reference a User
It feels a bit to magical that an instance variable that is being used in the views is generated by a private method here. I would probably suggest something like:

```ruby
def show
  @match = find_match
end

private

def find_match
  Match.find(params[:id])
end
```