Extract method to display a goal code and title

This is a pattern we're going to use in many places.

app/components/sdg_management/targets/index_component.html.erb

@@ -13,7 +13,7 @@
       <% targets.group_by(&:goal).map do |goal, targets| %>
         <tr class="goal-header">
           <th id="<%= header_id(goal) %>" colspan="2" scope="colgroup">
-            <%= goal.code %>. <%= goal.title %>
+            <%= goal.code_and_title %>
           </th>
         </tr>
 

app/models/sdg/goal.rb

@@ -16,4 +16,8 @@ class SDG::Goal < ApplicationRecord
   def self.[](code)
     find_by!(code: code)
   end
+
+  def code_and_title
+    "#{code}. #{title}"
+  end
 end