Players are propelled forward when slipping (#7886)

* Slippery System checks if players are already down before slipping them again. Prevents chains of banana peels accelerating players to supersonic speeds.

* Slippery items and puddles now propel people, some more than others.

* a white space

Content.Shared/Slippery/SharedSlipperySystem.cs

@@ -91,6 +91,11 @@ namespace Content.Shared.Slippery
                 return false;
             }
 
+            if (EntityManager.HasComponent<KnockedDownComponent>(otherBody.Owner))
+            {
+                return false;
+            }
+
             var ev = new SlipAttemptEvent();
             RaiseLocalEvent(otherBody.Owner, ev, false);
             if (ev.Cancelled)