28mm Wargame Billboard

Updated the Base fitting for 0.5mm clearance around the pylon so it hopefully slides into place without needing modification. Also shifted the borders so that the Billboard_Frames will but up against it help with gluing in place. Updated Billboard_Base.stl and 28mm_Billboard.stl

Remodel of the 28mm Billboard by [VexingCarcass](https://www.thingiverse.com/thing:3033305) to be more printer friendly. Not quite as pretty with square ladders, but is broken down into parts and laid out so there