[furaffinity] Add 'thumbnail' (#1284) and 'folders' properties

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.

gallery_dl/extractor/furaffinity.py

@@ -113,6 +113,9 @@ class FuraffinityExtractor(Extractor):
             data["gender"] = rh(extr('>Gender</strong>', '</div>'))
             data["width"] = pi(extr("<span>", "x"))
             data["height"] = pi(extr("", "p"))
+            data["folders"] = [rh(folder) for folder in extr(
+                '<h3>Listed in Folders</h3>',
+                '</section>').split('</a>') if rh(folder) != '']
         else:
             # old site layout
             data["title"] = text.unescape(extr("<h2>", "</h2>"))
@@ -132,11 +135,16 @@ class FuraffinityExtractor(Extractor):
             data["_description"] = extr(
                 '<td valign="top" align="left" width="70%" class="alt1" '
                 'style="padding:8px">', '                               </td>')
+            data["folders"] = []  # folders not present in old layout
 
         data["artist_url"] = data["artist"].replace("_", "").lower()
         data["user"] = self.user or data["artist_url"]
         data["date"] = text.parse_timestamp(data["filename"].partition(".")[0])
         data["description"] = self._process_description(data["_description"])
+        data["thumbnail"] = (
+            'https://t.furaffinity.net/' +
+            str(data['id']) + '@600-' +
+            data['url'].split('/')[-2].split('.')[0] + '.jpg')
 
         return data