When no cache store is set use cookie storage for sessions

A change in the default configuration of development environments
in rails 5.1 means that caching is now disabled by default so in
that case fall back to using cookies instead.

Closes openstreetmap#1666

config/initializers/session_store.rb

@@ -2,6 +2,8 @@
 
 if defined?(MEMCACHE_SERVERS)
   Rails.application.config.session_store :mem_cache_store, :memcache_server => MEMCACHE_SERVERS, :namespace => "rails:session", :key => "_osm_session"
-else
+elsif Rails.application.config.cache_store != :null_store
   Rails.application.config.session_store :cache_store, :key => "_osm_session"
+else
+  Rails.application.config.session_store :cookie_store, :key => "_osm_session"
 end