[FIX] point_of_sale: control if a cashier is in localstorage

Before this commit, we could temporarily change cashier from A to B, and back to A seamlessly just by refreshing the page.
This could be a security issue for a cashier is held responsible for its sales.

This commit stores the cashier in localstorage, and with the right sets of methods, corrects the issue.

OPW 767827

closes #19207

addons/point_of_sale/static/src/js/db.js

@@ -500,6 +500,13 @@ var PosDB = core.Class.extend({
         }
         return orders;
     },
+    set_cashier: function(cashier) {
+        // Always update if the user is the same as before
+        this.save('cashier', cashier);
+    },
+    get_cashier: function() {
+        return this.load('cashier');
+    }
 });
 
 return PosDB;

addons/point_of_sale/static/src/js/models.js

@@ -240,6 +240,7 @@ exports.PosModel = Backbone.Model.extend({
             }
 
             self.db.set_uuid(self.config.uuid);
+            self.cashier = self.get_cashier();
 
             var orders = self.db.get_orders();
             for (var i = 0; i < orders.length; i++) {
@@ -579,11 +580,12 @@ exports.PosModel = Backbone.Model.extend({
 
     // returns the user who is currently the cashier for this point of sale
     get_cashier: function(){
-        return this.cashier || this.user;
+        return this.db.get_cashier() || this.cashier || this.user;
     },
     // changes the current cashier
     set_cashier: function(user){
         this.cashier = user;
+        this.db.set_cashier(this.cashier);
     },
     //creates a new empty order and sets it as the current order
     add_new_order: function(){