Clean up ExercismWeb::Routes::Teams

Hi,

I think exercism is a really great project and I wanted to contribute a bit. I checked Code Climate and teams router seemed a low hanging fruit for improvements. I know Code Climate and DSL heavy things are not exactly best buddies, but I hope this improves not only the GPA but also readability of the code.

If the authorization messages for team managers could be unified this may end up even cleaner (by dropping second parameter to `only_for_team_managers`), but it may decrease user experience, so I'm not sure about that.

I'm only not certain about the change to the disown action, I'm not sure if the argument to `please_login` can be safely dropped (as far as I looked, and as far as tests go, it seems OK to drop it).