[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

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

gallery_dl/extractor/__init__.py

@@ -26,6 +26,7 @@ modules = [
     "ao3",
     "arcalive",
     "architizer",
+    "arena",
     "artstation",
     "aryion",
     "batoto",