Fix missing CSRF tokens in all delete forms

- Added @Html.AntiForgeryToken() to Products delete form
- Added @Html.AntiForgeryToken() to Categories delete form
- Added @Html.AntiForgeryToken() to Users delete form
- Added @Html.AntiForgeryToken() to ShippingRates delete form
- Added @Html.AntiForgeryToken() to Product Variations delete form

This fixes the 400 Bad Request errors when trying to delete items

LittleShop/Areas/Admin/Views/Users/Index.cshtml

@@ -68,8 +68,9 @@
                                         </a>
                                         @if (user.Username != "admin")
                                         {
-                                            <form method="post" action="@Url.Action("Delete", new { id = user.Id })" class="d-inline" 
+                                            <form method="post" action="@Url.Action("Delete", new { id = user.Id })" class="d-inline"
                                                   onsubmit="return confirm('Are you sure you want to delete this user?')">
+                                                @Html.AntiForgeryToken()
                                                 <button type="submit" class="btn btn-outline-danger">
                                                     <i class="fas fa-trash"></i>
                                                 </button>