conditionally showing "delete" link on Accounts page

Sinatra:

- moving edit and delete route-creation code into controller

- adding is_admin? method-stub to helpers - TODO: implement

Rails:

- adding @delete_enabled into users_controller. value is true if user_id on session corresponds to a Stormpath account associated with group named "admin"

-
Implementing is_admin? in Sinatra app

- adding is_authenticated? helper

- adding initialize and destroy_session methods which try to hide the @is_admin setting/destruction from the route-handlers
Conditionally disabling the "Delete" button in Rails
Merge branch 'enhancements/accounts_listing_check_admin_group-12' into development