/datum/atom_hud (Passive HUDs) bugfixes

Moves antag_hud image and datum to mind instead of mob.

Fixes mobs keeping data HUDs after transfer_to().
Fixes gibbed bodies losing their antag HUDs.
Fixes the data hud disappearing when dropping unrelated HUDs.

Fixes #6206
Fixes #6209
CRASH: Crashes the current procedure, displaying the specified message and generating diagnostic debugging output

Not the server, just the current proc.