v772: load airbrake and insert middleware on staging/production (#1144)

This will show the airbrake ID to end-users for easier followup.

To test locally:
 - set AIRBRAKE_API_KEY
 - remove if staging/production checks in application.rb
 - uncomment config.development_environments
 - raise an exception in a controller
 - set config.consider_all_requests_local = false