Rename "anonymous" tag

BUR #11591 has been approved by @evazion.

rename anonymous -> anonymous_(4chan)

The tag wiki of the character anonymous said that it was meant to be used for 4chan character, yet it's currently used for generic faceless characters. I think this tag should be renamed to avoid confusion.

I think that's too specific/noninclusive to other anons, such as from tumblr. Wouldn't it make more sense to change the wiki entry?

Other sites can get their own qualified tags.

The bulk update request #11591 (forum #218301) has been approved by @evazion.