Fanny_pack utility alias

BUR #41911 has been rejected.

create alias sling_bag -> fanny_pack

In danbooru, both refer to the same style of bag. A bag that is worn over one shoulder and across the chest/back.

As long as people don't confuse it with shoulder bag.

The bulk update request #41911 (forum #358632) has been rejected by @nonamethanks.

Wrote a disambiguation wiki, I'd say it's too risky an alias.