Refactor pair_retro :create action to use Canary

[mbramson]

The pair_retro create route is reimplementing a lot of logic that should in theory be handled by Canary. Evaluate whether this code can be improved through the use of Canary.

Some fiddling reveals that this is probably a bit more complicated than simply adding the :create action to the load_and_authorize_resource, as the Canada protocol doesn't seem to pick up the user_id of the retro, as it is passed in as a parameter nested in the pair_retro parameter. So conn.assigns.authorized will be true for admins, but not when the passed in nested user_id parameter matches the user id of the logged in user in .conn.assigns.current_user.

This might be as simple as figuring out what to match on in this case in the Canada protocol, or it may involve some more in depth logic. Could be a good opportunity for a PR to Canada or Canary.

It's also possible that the solution is actually less elegant than the currently existing code. In which case this would be closed with no code change.

[mbramson]

Closing this for now, as after some further investigation, this is a pretty substantial change to canary, and the a lot of the complexity in this method will be removed once we move to Phoenix 1.3 and get FallBackControllers.