Fixes  #5532 and increases the time required for organs to become infected from wounds

Also removes the now unnecessary get_cure_threshold() proc, caps the
rate at which an organ can receive germs from wounds, and makes germs
spread from external to internal organs happen one organ at a time
instead of all at once.

(cherry picked from commit a4c9f31b493cfa5cb388afa15399230d48953ddc)

Conflicts:
	code/setup.dm
Makes minor wounds harder to infect

(cherry picked from commit 5764d958082f1a2406e243bc5e30eefacb04840b)
Tweaks internal organ spread rate

(cherry picked from commit 008c5447b7bd39feac3ca19323a2337fc24bde36)
Increases cough rate for lung infections

(cherry picked from commit b8a102beb4a545936f38e60b0e538673b5566693)
Fixes missing length check before pick()

(cherry picked from commit dcfbeb092433d0e3f5f579ab9de627946edeab59)