Allow target and local target comparison

This way we'll be able to sort a mixed collection of targets and local
targets.

app/models/sdg/local_target.rb

@@ -17,10 +17,12 @@ class SDG::LocalTarget < ApplicationRecord
 
   belongs_to :target
 
-  def <=>(local_target)
-    return unless local_target.class == self.class
-
-    [target, numeric_subcode] <=> [local_target.target, local_target.numeric_subcode]
+  def <=>(any_target)
+    if any_target.class == self.class
+      [target, numeric_subcode] <=> [any_target.target, any_target.numeric_subcode]
+    elsif any_target.class == target.class
+      -1 * (any_target <=> self)
+    end
   end
 
   protected

app/models/sdg/target.rb

@@ -12,10 +12,12 @@ class SDG::Target < ApplicationRecord
     I18n.t("sdg.goals.goal_#{goal.code}.targets.target_#{code_key}.title")
   end
 
-  def <=>(target)
-    return unless target.class == self.class
-
-    [goal.code, numeric_subcode] <=> [target.goal.code, target.numeric_subcode]
+  def <=>(any_target)
+    if any_target.class == self.class
+      [goal.code, numeric_subcode] <=> [any_target.goal.code, any_target.numeric_subcode]
+    elsif any_target.class.name == "SDG::LocalTarget"
+      [self, -1] <=> [any_target.target, 1]
+    end
   end
 
   def self.[](code)