Manifest ghost visibility-toggle

Ghosts which have been made visible by cultists can afterward turn themselves (in)visible at will, with a cooldown before being able to turn themselves visible after having faded from sight.
Merge remote-tracking branch 'upstream/dev' into Ghosts

Conflicts:
	code/modules/mob/dead/observer/observer.dm
Merge pull request #6943 from PsiOmegaDelta/Ghosts

Manifest ghost visibility-toggle