Two small patches relating to sessions

I have added two small patches to Lighthouse relating to sessions. The first removes the very old #reloadable? method from ActiveRecord::SessionStore. The second updates the config/initializers/session_store.rb file to use the new :key option in the session configuration instead of the old CGI :session_key option. The patch also updates related documentation and messages referring to the old :session_key option.

[1] #1745 Remove unused reloadable? method from ActiveRecord::SessionStore - Ruby on Rails - rails [2] #1746 Use :key instead of old :session_key in session_store.rb generator and docs - Ruby on Rails - rails