[e621] Support for e621.cc and e621.anthro.fr

[ClosedPort22]

e621.cc and e621.anthro.fr are alternative frontends to e621 with better mobile support. It'd be nice if gallery-dl could recognize these domains as aliases to e621.net.

But there's a problem - both sites have nonstandard URL patterns:

https://e621.net/posts?tags=rating%3Asafe
https://e621.anthro.fr/?q=rating%3Asafe
https://e621.cc/?tags=rating%3Asafe

And although I can add alternative domains to a given instance through pattern, it doesn't seem possible to change the path or query parameter without affecting other instances as well.

I managed to work around this by adding a "proxy" extractor, like so:

class E621FrontendExtractor(Extractor):
    """Extractor for alternative frontends for e621"""
    basecategory = "E621"
    subcategory = "frontend"
    pattern = r"(?:https?://)?(?:e621\.cc/\?tags|e621\.anthro\.fr/\?q)=([^&#]*)"
    example = "https://e621.cc/?tags=TAG"

    def __init__(self, match):
        Extractor.__init__(self, match)
        self.tags = match.group(1)

    def items(self):
        yield Message.Queue, "https://e621.net/posts?tags=" + self.tags, {"_extractor": E621TagExtractor}

But this seems kind of hacky to me, and I'm at a loss for a better solution.

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As always, thank you for all the work you have poured into developing and maintaining gallery-dl, @mikf !