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/ShippingRates/Index.cshtml

@@ -62,8 +62,9 @@
                                         <a href="@Url.Action("Edit", new { id = rate.Id })" class="btn btn-outline-primary">
                                             <i class="fas fa-edit"></i>
                                         </a>
-                                        <form method="post" action="@Url.Action("Delete", new { id = rate.Id })" class="d-inline" 
+                                        <form method="post" action="@Url.Action("Delete", new { id = rate.Id })" class="d-inline"
                                               onsubmit="return confirm('Are you sure you want to delete this shipping rate?')">
+                                            @Html.AntiForgeryToken()
                                             <button type="submit" class="btn btn-outline-danger">
                                                 <i class="fas fa-trash"></i>
                                             </button>