Feature/flutter mobile task 4 create an experiment called item sorting

[lttsNilesh]

1. Create an experiment called item_sorting
   a. Add 3 groups: control, test oldest, test newest
   b. Add a single parameter sort_order, which is
   “default”, “newest_first” or “oldest_first”
   c. Implementing the different sorting by checking
   the experiment and parameter value for the user
2. Add a simple account system with a login. When the user is logged out, do not set a userID on the user. When the user logs in, set the userID as the logged in userID.
   a. On mobile, you will need to manually call updateUser when the login state changes and handle waiting for the response before showing the list of items