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?
FYI, this `conditions` parameter is deprecated and will have to be yanked out before an upgrade to 4.0

I think the recommended way is:

    has_many :discussable_groups,
      -> { where(members_can_start_discussions: true, archived_at: nil) }, 
      through: memberships, #etc