Enable shopping cart, add items to cart, enable tips (#410)

Modal cart, remove items, checkout

Fix removal and adding of cart items

Improve cart UI

Add cart bundle, remove unused js files from the view when cart isn't used

Do not enable cart by default

Do not put modal into the view when the cart is disabled

Escape js properties

Work with amounts as cents

Make animation speed look constant

Enable tips in the cart

Fix cart UI

BTCPayServer/Models/AppViewModels/UpdatePointOfSaleViewModel.cs

@@ -17,6 +17,8 @@ namespace BTCPayServer.Models.AppViewModels
         [MaxLength(5000)]
         public string Template { get; set; }
 
+        [Display(Name = "Enable shopping cart")]
+        public bool EnableShoppingCart { get; set; }
         [Display(Name = "User can input custom amount")]
         public bool ShowCustomAmount { get; set; }
         public string Example1 { get; internal set; }
@@ -32,6 +34,10 @@ namespace BTCPayServer.Models.AppViewModels
         [MaxLength(30)]
         [Display(Name = "Text to display on buttons next to the input allowing the user to enter a custom amount")]
         public string CustomButtonText { get; set; }
+        [Required]
+        [MaxLength(30)]
+        [Display(Name = "Do you want to leave a tip?")]
+        public string CustomTipText { get; set; }
 
         [MaxLength(500)]
         [Display(Name = "Custom bootstrap CSS file")]