Splitting arm_on_head

BUR #21115 has been approved by @nonamethanks.

create alias arm_on_head -> arm_on_own_head
remove alias arm_on_head -> arm_on_own_head
deprecate arm_on_head

Went through and manually cleaned this out after discovering arm_on_another's_head only had 12 posts. Goal is to replicate hand_on_another's_head and hand_on_own_head.

This bulk update request is pending automatic rejection in 5 days.

This bulk update request has been rejected because it was not approved within 45 days.

The bulk update request #21115 (forum #259581) has been rejected by @DanbooruBot.

What was the issue here?

The bulk update request #21115 (forum #259581) has been approved by @nonamethanks.