Allow content to control what field and screens it is displayed in.

Content classes now include a can_display_in? method which contains the logic we use to see if a piece of content should be rendered in a specific <screen, field>.  To provide parity with the default behavior, we simply make sure that the Field.Kind == Content.Kind.

app/models/content.rb

@@ -222,4 +222,11 @@ def self.filter_all_content(params)
     filtered_contents
   end
 
+  # Determine if a piece of content should be displayed based on screen and field.
+  # By default content can be rendered in any Field which has the same Kind as the Content.
+  # This can be overridden by different content types which can be displayed in different
+  # fields or based on some dynamic criteria implemented in each Content subclass.
+  def can_display_in?(screen, field)
+    return self.kind == field.kind
+  end
 end

app/models/subscription.rb

@@ -32,6 +32,8 @@ def weight_name
 
   # Get an array of all the approved active content to be shown in a screen's field.
   def contents
-    self.feed.approved_contents.active.where(:kind_id => self.field.kind.id)
+    contents = self.feed.approved_contents.active.all
+    contents.reject!{|c| !c.can_display_in?(self.screen, self.field)}
+    return contents
   end
 end