Retrieve 'thumbnail' and 'folders' properties for each post.
'thumbnail' (#1284):
- Preview image used for search results, writing posts, music, etc.
- Filename format: <post_id>@600-<directory_containing_full_image>.jpg
'Folders' (related to #1817):
- A list of all gallery folders containing this post
- Folder name format: [<folder_category> - ]<folder_name>
- Only works on new layout; old layout does not show folders, so list will be empty
A test is included for each property.
