Make Pay Button a plugin (#3862)

* Move files

* Fix potentially missing default payment method

Before, it got removed if any other value was changed besides the default payment method.

* Fix missing store data

* Update BTCPayServer/Plugins/PayButton/PayButtonPlugin.cs

Co-authored-by: Pavlenex <pavle@pavle.org>

* Update pay button warning

Closes #3535.

Co-authored-by: Pavlenex <pavle@pavle.org>

BTCPayServer/Plugins/PayButton/Models/PayButtonViewModel.cs

@@ -0,0 +1,54 @@
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using BTCPayServer.ModelBinders;
+using BTCPayServer.Models.AppViewModels;
+using BTCPayServer.Models.StoreViewModels;
+using Microsoft.AspNetCore.Mvc;
+
+namespace BTCPayServer.Plugins.PayButton.Models
+{
+    public class PayButtonViewModel
+    {
+        [ModelBinder(BinderType = typeof(InvariantDecimalModelBinder))]
+        public decimal? Price { get; set; }
+        public string InvoiceId { get; set; }
+        [Required]
+        public string Currency { get; set; }
+        public string DefaultPaymentMethod { get; set; }
+        public PaymentMethodOptionViewModel.Format[] PaymentMethods { get; set; }
+        public string CheckoutDesc { get; set; }
+        public string OrderId { get; set; }
+        public int ButtonSize { get; set; }
+        public int ButtonType { get; set; }
+
+        // Slider properties (ButtonType = 2)
+        public decimal Min { get; set; }
+        public decimal Max { get; set; }
+        public string Step { get; set; }
+
+        // Custom Amount properties (ButtonType = 1)
+        public bool SimpleInput { get; set; }
+        public bool FitButtonInline { get; set; }
+
+        [Url]
+        public string ServerIpn { get; set; }
+        [Url]
+        public string BrowserRedirect { get; set; }
+        [EmailAddress]
+        public string NotifyEmail { get; set; }
+
+        public string StoreId { get; set; }
+        public string CheckoutQueryString { get; set; }
+
+        // Data that influences Pay Button UI, but not invoice creation
+        public string UrlRoot { get; set; }
+        public List<string> CurrencyDropdown { get; set; }
+        public string PayButtonImageUrl { get; set; }
+        public string PayButtonText { get; set; }
+        public bool UseModal { get; set; }
+        public bool JsonResponse { get; set; }
+        public ListAppsViewModel.ListAppViewModel[] Apps { get; set; }
+        public string AppIdEndpoint { get; set; } = "";
+        public string AppChoiceKey { get; set; } = "";
+    }
+}