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.
I can, but that would be lazy, unsafe (because it would fail silently instead of throwing a runtime if it was called on a non-human) and would compound the already great issue of the lack of proper mob typecasting all over the code, which is one of the things that makes it hard to perform a proper mob refactor.