perf: memoize member organiser status

This is very unlikely to change during one request, so we can memoise it and gain some speed

Author: mroderick

app/presenters/member_presenter.rb

@@ -1,6 +1,6 @@
 class MemberPresenter < BasePresenter
   def organiser?
-    has_role? :organiser, :any
+    @organiser ||= has_role? :organiser, :any
   end
 
   def event_organiser?(event)