[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>

scripts/supportedsites.py

@@ -30,6 +30,7 @@ CATEGORY_MAP = {
     "ao3"            : "Archive of Our Own",
     "archivedmoe"    : "Archived.Moe",
     "archiveofsins"  : "Archive of Sins",
+    "arena"          : "Are.na",
     "artstation"     : "ArtStation",
     "aryion"         : "Eka's Portal",
     "atfbooru"       : "ATFBooru",