set username on email change

closes #673

BTCPayServer/Controllers/ManageController.cs

@@ -111,6 +111,7 @@ namespace BTCPayServer.Controllers
                 var setEmailResult = await _userManager.SetEmailAsync(user, model.Email);
                 if (!setEmailResult.Succeeded)
                 {
+                    await _userManager.SetUserNameAsync(user, model.Username);
                     throw new ApplicationException($"Unexpected error occurred setting email for user with ID '{user.Id}'.");
                 }
             }