Build 67: reworked knockback system

What’s new:

• Reworked knockback system: KncobackForceMultiplier is gone, now there’s KnockbackSpeed that can be either null (or absent) so the weapon ha no knockback, set to zero so the weapon stops & blocks enemy movement but doesn’t cause knockback, or a value that moves the enemy like MovementSpeed, but in the knockback’s direction.
• The CodeMirror based rule editor can be removed more easily, just by deleting “_main.js”.
• Some random balancing: decreased player health & speed.