/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
Yeah, but this really isn't the end of the world (everything will work fine unless the non-living mob's mind changes mobs), so I don't think it's worth crashing the server for this.