Avoid conflict between SDG::Manager classes

On production environments the application wasn't loading because
the `SDG::Manager` class was defined in two places. We don't know
the exact reasons for the conflict and why these changes fix it.

app/models/abilities/sdg/manager.rb

@@ -1,12 +1,10 @@
-module Abilities
-  class SDG::Manager
-    include CanCan::Ability
+class Abilities::SDG::Manager
+  include CanCan::Ability
 
-    def initialize(user)
-      merge Abilities::Common.new(user)
+  def initialize(user)
+    merge Abilities::Common.new(user)
 
-      can :read, ::SDG::Goal
-      can :read, ::SDG::Target
-    end
+    can :read, ::SDG::Goal
+    can :read, ::SDG::Target
   end
 end