Y-belt holder and tensioner for MK3 and Bear MK2, MK2.5, and MK3

Motivation
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There's a lot to love about the Prusa printers, but the Y axis belt line is not one of them. A 16T pulley on the motor, a 20T pulley on the idler, and an offset going through the belt holder introduce Z axis forces on your bed as it traverses along Y. These parts allow you to use a 16T idler and get a perfectly parallel belt line, keeping the bed more level as it travels.

User [RHDreambox](https://shop.prusa3d.com/forum/memberlist.php?mode=viewprofile&u=42061) designed [