Starts laying out flow and views for adding Sales to a Customer
Adds SalesApp.Edit module

* When clicking Add Purchase from the CustomersApp.Show view, a new Sale
  is created and we route the user to Edit the new Sale.
* Added functionality to destroy this newly created Sale from inside
  SalesApp.Edit view
Adds after_save and after_destroy hooks on LineItem model.
Adds Api::LineItemsController to handle CRUD operations

* A user can create, update, destroy, show operations on LineItems
* This feature will come in handy when creating Sales from the JS app.
* Adds new relationships from User -> Customer -> Sale -> LineItem
Adds AccountSetting model to store user specific defaults

* A user can store preferred interface language
* A user can store his/her store name