Makes data huds passive

Previously, HUDs updated the HUD images for every single mob in view() of a HUD user every single tick.

With this update, all the HUD will do is add all mob HUD images to the client. Image updating is now handled by a series of hooks included in data_hud.dm and called as needed by other procs.

This should make HUDs more responsive and less laggy. This has been somewhat tested, however there are bound to be bugs because it relies on certain procs being called whenever stuff like health or arrest status has changed. I tried to catch 'em all but there's probably some I missed.

Partially addresses #4040 (does not fix completely because antag huds are untouched)
This arbitrarily limits HUD's to humans only, while this makes sense given the current codebase, I would argue that moving them futher up the tree would be more sensible - I don't think it should reach all the way to atom, but maybe define at mob level.

That way future coders can easily add huds for non human mobs.