Sign in with other device (quick mobile login) (#2504)

Co-authored-by: Dennis Reimann <mail@dennisreimann.de>
Co-authored-by: Nicolas Dorier <nicolas.dorier@gmail.com>

BTCPayServer/Controllers/ManageController.LoginCodes.cs

@@ -0,0 +1,21 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc;
+
+namespace BTCPayServer.Controllers
+{
+    public partial class ManageController
+    {
+        [HttpGet]
+        public async Task<IActionResult> LoginCodes()
+        {
+            var user = await _userManager.GetUserAsync(User);
+            if (user == null)
+            {
+                throw new ApplicationException($"Unable to load user with ID '{_userManager.GetUserId(User)}'.");
+            }
+
+            return View(nameof(LoginCodes), _userLoginCodeService.GetOrGenerate(user.Id));
+        }
+    }
+}