Store: Move support URL to Checkout Appearance and improve wording (#5717)

As discussed in the recent design meeting.

BTCPayServer/Models/StoreViewModels/CheckoutAppearanceViewModel.cs

@@ -76,6 +76,10 @@ namespace BTCPayServer.Models.StoreViewModels
         [Display(Name = "Custom HTML title to display on Checkout page")]
         public string HtmlTitle { get; set; }
 
+        [Display(Name = "Support URL")]
+        [MaxLength(500)]
+        public string SupportUrl { get; set; }
+
         [Display(Name = "Show a timer … minutes before invoice expiration")]
         [Range(1, 60 * 24 * 24)]
         public int DisplayExpirationTimer { get; set; }