Tweak admin UI templates (#25)

- Move user management under web log settings
- Move user self-update to my-info
- Return meaningful error if a template does not exist
- Tweak margins/paddings throughout
- Do not show headings on list pages if lists are empty
- Fix pagination styles for page/post list pages

src/admin-theme/theme-upload.liquid

@@ -1,5 +1,5 @@
 <div class="col">
-  <h5>{{ page_title }}</h5>
+  <h5 class="mt-2">{{ page_title }}</h5>
   <form action="{{ "admin/theme/new" | relative_link }}" method="post" class="container" enctype="multipart/form-data"
         hx-boost="false">
     <input type="hidden" name="{{ csrf.form_field_name }}" value="{{ csrf.request_token }}">