Add PublicBody#calculated_home_page!

Removes the TODO by adding a cached and non-cached version of
PublicBody::CalculatedHomePage#calculated_home_page.

In current use it's safe to always use the cached version.

In future we might want to modify the non-bang version such that it
recalculates if the request_email attribute gets changed.

app/models/public_body/calculated_home_page.rb

@@ -22,12 +22,15 @@ module PublicBody::CalculatedHomePage
     ]
   end
 
+  def calculated_home_page
+    @calculated_home_page ||= calculated_home_page!
+  end
+
+  private
+
   # Guess home page from the request email, or use explicit override, or nil
   # if not known.
-  #
-  # TODO: PublicBody#calculated_home_page would be a good candidate to cache
-  # in an instance variable
-  def calculated_home_page
+  def calculated_home_page!
     if home_page && !home_page.empty?
       home_page[URI.regexp(%w(http https))] ? home_page : "https://#{home_page}"
     elsif request_email_domain
@@ -36,8 +39,6 @@ def calculated_home_page
     end
   end
 
-  private
-
   def excluded_calculated_home_page_domain?(domain)
     excluded_calculated_home_page_domains.include?(domain)
   end