Add meta item

- Use meta item for category/author lookups
- Add Liquid filter to get values from meta item lists
- Implement log on "returnUrl" parameter

src/MyWebLog/themes/admin/log-on.liquid

@@ -2,6 +2,9 @@
 <article class="py-3">
   <form action="/user/log-on" method="post">
     <input type="hidden" name="{{ csrf.form_field_name }}" value="{{ csrf.request_token }}">
+    {% if model.return_to %}
+      <input type="hidden" name="returnTo" value="{{ model.return_to.value }}">
+    {% endif %}
     <div class="container">
       <div class="row pb-3">
         <div class="col col-md-6 col-lg-4 offset-lg-2">

src/MyWebLog/themes/admin/post-list.liquid

@@ -31,7 +31,7 @@
               <a href="#" class="text-danger">Delete</a>
             </small>
           </td>
-          <td>{{ post.author_name }}</td>
+          <td>{{ model.authors | value: post.author_id }}</td>
           <td>{{ post.status }}</td>
           <td>{{ post.tags | join: ", " }}</td>
         </tr>