Make wallet able to send to multiple destinations (#847)

* Make wallet able to send to multiple destinations

* fix tests

* update e2e tests

* fix e2e part 2

* make headless again

* pr changes

* make wallet look exactly as old one when only 1 dest

BTCPayServer/Extensions/ModelStateExtensions.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Linq.Expressions;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+using Microsoft.AspNetCore.Mvc.ViewFeatures.Internal;
+
+namespace BTCPayServer
+{
+    public static class ModelStateExtensions
+    {
+        public static void AddModelError<TModel, TProperty>(
+            this ModelStateDictionary modelState, 
+            Expression<Func<TModel, TProperty>> ex, 
+            string message
+        )
+        {
+            var key = ExpressionHelper.GetExpressionText(ex);
+            modelState.AddModelError(key, message);
+        }
+        
+        public static void AddModelError<TModel, TProperty>(this TModel source,        
+            Expression<Func<TModel, TProperty>> ex, 
+            string message,
+            ModelStateDictionary modelState)
+        {
+            var key = ExpressionHelper.GetExpressionText(ex);
+            modelState.AddModelError(key, message);
+        }
+    }
+}