Sync shopping basket 'state' with 'database' in ESAP-GUI

Expand the 'write only' ShoppingBasket functionality to an interactive functionality where the contents of the ShoppingBasket in the database are synchronized with the internal ShoppingBasket in the frontend in a clear and predictable way.

[ ] = todo: [x] = done

• when the users authenticate, the contents of their shopping basket should be loaded. With some info about that visible in the GUI
• when the user changes the selection in the shoppingbasket, it should be clear in the GUI that it is 'unsaved'.
• the user should be able to save the contents of the shopping basket, which should be seen in GUI.
• the user should be able to remove items from the shopping basket