Implement tag mapping

- Move all admin functions to /admin URLs
- Create Liquid filters for page/post edit, category/tag link
- Update all themes to use these filters
- Add delete for pages/posts
- Move category/page functions to Admin module

src/MyWebLog/themes/admin/category-edit.liquid

@@ -1,6 +1,6 @@
 <h2 class="my-3">{{ page_title }}</h2>
 <article>
-  <form action="/category/save" method="post">
+  <form action="/admin/category/save" method="post">
     <input type="hidden" name="{{ csrf.form_field_name }}" value="{{ csrf.request_token }}">
     <input type="hidden" name="categoryId" value="{{ model.category_id }}">
     <div class="container">