Anet E12; Z-axis sync screws and run on one stepper motor and reduce bed leveling.

I remixed the design to work with the E12 and to improve upon it.
1. tightening only on one side of the belt can cause issues with turning the lead screws in opposite directions thus causing the heights to change from left to right, one will go up, and the other will go down. My design tightens on both sides of the timing belt.
2. Since mine tightens on both sides at once, it engages more of the teeth on the pulleys on the lead screws; this helps to make it possible to run off one z-axis stepp