Update SDG header component

* Add a header element as component markup wrapper
* Allow component to receive an optional block
* Add reusable styles for header links

Co-autored-by: Javi Martín <javim@elretirao.net>

app/components/sdg_management/header.rb

@@ -1,11 +1,17 @@
 module SDGManagement::Header
   extend ActiveSupport::Concern
 
-  def header
+  def header(&block)
     provide(:title) do
       "#{t("sdg_management.header.title")} - #{title}"
     end
 
-    tag.h2 title
+    tag.header do
+      if block_given?
+        tag.h2(title) + capture(&block)
+      else
+        tag.h2(title)
+      end
+    end
   end
 end