Add id for register form

Author: HaoK

src/Identity/UI/src/Areas/Identity/Pages/V4/Account/Register.cshtml

@@ -8,7 +8,7 @@
 
 <div class="row">
     <div class="col-md-4">
-        <form asp-route-returnUrl="@Model.ReturnUrl" method="post">
+        <form id="register" asp-route-returnUrl="@Model.ReturnUrl" method="post">
             <h4>Create a new account.</h4>
             <hr />
             <div asp-validation-summary="All" class="text-danger"></div>