[arena] add 'channel' extractor (#5847 #8509)

* feat: issue 5847 site support for are.na
* flake8 lint error fix
* class name error fix
* update
    - prevent unnecessary request to 'page_url'
    - fix pagination
    - simplify block extraction code
    TODO: - rewrite without GalleryExtractor
          - extractors for Blocks, Users, etc
* supportedsites
* tests
* rename to 'channel' extractor
* update site title to 'Are.na'
* prioritize attachments

---------

Co-authored-by: Mike Fährmann <mike_faehrmann@web.de>

docs/supportedsites.md

@@ -109,6 +109,12 @@ Consider all listed sites to potentially be NSFW.
     <td>Search Results, Series, Subscriptions, Tag Searches, User Profiles, User Bookmarks, User Series, User Works, Works</td>
     <td>Supported</td>
 </tr>
+<tr id="arena" title="arena">
+    <td>Are.na</td>
+    <td>https://are.na/</td>
+    <td>Channels</td>
+    <td></td>
+</tr>
 <tr id="artstation" title="artstation">
     <td>ArtStation</td>
     <td>https://www.artstation.com/</td>