Memoize goal_text in SprintComponent

Avoids repeated `goal_text_for` lookups when the goals association is
not preloaded.

Author: myabc

modules/backlogs/app/components/backlogs/sprint_component.rb

@@ -92,7 +92,9 @@ def finish_sprint_button_arguments
     end
 
     def goal_text
-      sprint.goal_text_for(project)
+      return @goal_text if defined?(@goal_text)
+
+      @goal_text = sprint.goal_text_for(project)
     end
 
     def sprint_goal_id