Added extension and screen relation associations

Necessary, now that a screen has a `extension_id` foreign ID, to allow a screen to belong to an extension while an extension can have many screens.

Issue: 360
Milestone: minor

app/relations/extension.rb

@@ -11,6 +11,7 @@ module Terminus
           has_many :extension_models, relation: :extension_model
           has_many :models, through: :extension_model, relation: :model, as: :models
           has_many :extension_exchanges, relation: :extension_exchange
+          has_many :screens, relation: :screen
         end
       end
     end

app/relations/screen.rb

@@ -7,6 +7,7 @@ module Terminus
       schema :screen, infer: true do
         associations do
           belongs_to :model, relation: :model
+          belongs_to :extension, relation: :extension
           belongs_to :template, relation: :screen_template
           has_many :playlist_items, relation: :playlist_item, as: :playlist_items, view: :ordered
           has_many :playlists, through: :playlist_item, relation: :playlist, as: :playlists