Makes data huds passive.
Data hud revamp improvements and bugsquashing

Adds a changelog for the HUD change.
Hud rewrite rewrite. Now datum-based and somewhat generic.
Datum-based huds! Lays down the framework not only for data huds but also for antag huds.
Includes antag HUDs in the datum hud overhaul

Makes hud_list associative, and introduces the /atom list hud_possible,
which lists all the possible hud images a certain atom can provide.

Replaces all the awful procs in the gamemodes with their new
/datum/atom_hud versions, which are much shorter, simpler and faster.
Fixes roundstart runtime involving HUDs
Merge pull request #5908 from tkdrg/hud

Makes data and antag huds passive and creates /datum/atom_hud