Add empty SDG homepage configuration page

Here we'll eventually display links to edit the homepage and the cards
in it.

app/controllers/sdg_management/homepage_controller.rb

@@ -0,0 +1,5 @@
+class SDGManagement::HomepageController < SDGManagement::BaseController
+  def show
+    @phases = SDG::Phase.accessible_by(current_ability).order(:kind)
+  end
+end