Add page position id method

This will be used to identify the corresponding id for the appropriate
element on the routes page matching the page in that position, so that
it can be anchored to by links going to that page.

Author: SamJamCul

app/models/page.rb

@@ -133,6 +133,10 @@ def normalise_welsh!
     routing_conditions.each(&:normalise_welsh!)
   end
 
+  def page_position_id
+    "page-#{position}"
+  end
+
 private
 
   def guidance_fields_presence

spec/models/page_spec.rb

@@ -872,4 +872,10 @@
       end
     end
   end
+
+  describe "#page_position_id" do
+    it "returns a string for the page position id" do
+      expect(page.page_position_id).to eq "page-#{page.position}"
+    end
+  end
 end