on /d/new, only display groups that you can start a discussion in

@robguthrie I'm pretty confident this is providing the intended functionality, but I wonder how you feel about the clarity of what I've done?
It would be way nicer to read, if you create all three groups at the start, then test against each of them. Mutating object like this will cause you problems in the long run. You only want to modify objects when you really really have to.