Disallow adding/editing of retrospectives for users no longer in the group

[mbramson]

Currently, if a user is no longer in a group, they can still edit retrospectives that they created while they were in that group. They can also add retrospectives for pairs they were in while they were in the group. This allows the user to view the group's projects (which they should not have access to if they are no longer in that group).

Users that are no longer in a group associated with a pair they were in should not be allowed to access the :new, :create, :edit, or :update routes for such a retrospective. They should be allowed to view the retrospective if they are no longer in the group, so they should be allowed to access the :show route for such a restrospective.