Add method to easily access a goal by code

Similar to what we do with settings, only for settings we return the
value of the setting (which is what we're going to need most of the
time), and here we return the object.

app/models/sdg/goal.rb

@@ -8,4 +8,8 @@ class SDG::Goal < ApplicationRecord
   def description
     I18n.t("sdg.goals.goal_#{code}.description")
   end
+
+  def self.[](code)
+    find_by!(code: code)
+  end
 end