Ultimaker 3 double spool holder

Extension of my ([original Ultimaker 3 spoolholder](https://www.thingiverse.com/thing:2742602)) to hold both spools.
This includes some slight modifications to the material 1 spool holder to make it 'click' on the back of the printer.

To print it, rotate it 90 degrees on their side, so it can be printed without support material.