Hand gesture is an ambiguous tag

BUR #25353 has been approved by @nonamethanks.

deprecate hand_gesture

According to hand gesture's wiki page:
"Ambiguous tag. Do not use."

This tag should be deprecated to prevent it's misuse, as Danbooru itself discourages usage of this tag.

I could see this being useful for people moving their hands while speaking, but yeah, as it stands it's a padding tag.

The bulk update request #25353 (forum #277534) has been approved by @nonamethanks.