Add helper method for filtering the sitemap for the current user

[tidyui]

This method should filter the given collection SitemapItem objects and only return the ones the current user has access to. This is to simplify menu rendering in applications with secured pages.

Example

@inject Microsoft.AspNetCore.Authorization.IAuthorizationService Auth

<ul>
   @foreach (var item in await WebApp.Site.Sitemap.ForUserAsync(User, Auth))
   {
      <li><a href="@item.Permalink">@item.MenuTitle</a></li>
   }
</ul>

This also includes adding the Permission collection to the SitemapItem which is currently missing.