Fix virus detectability for medbot and medical HUD

* Medbots will stop detecting viruses undetectable to Health Analyzers or Pandemic machines.
* Medbots will stop considering 100% harmless(beneficial or neutral symptoms) viruses as a disease to cure.
* gives a severity to all standard diseases.
* gives a severity var to symptoms.
* Players with only harmless (beneficial or neutral) viruses don't appear as ill on medical HUD.

Fixes #3052