HyperCube Evolution improved double Z axis linear rail bracket

The original bracket flange breaks easily over the wide 8 mm slot of the 3030 profile. This inspired me to improve the brackets by locally reinforcing the print with an alternative infill percentage (the insert STL need to be used for this) and a washer to bridge the extrusion slot gap.

Improved reinforced double Z axis linear rail top brackets, improvements:

- Added an insert object that can be used in the slicer to have [100 % local infill](https://3dprinting.stackexchange.com/a/6523) to prevent crushing of the bracket flange
- Slot for a 15 mm washer to prevent cracking of the lips of the flange (distributes the load better over the 3030 extrusion slot)
- Tapered access hole to allow easier access to the Allen key bolt