Sentinel Picatinny Rails Printable

Remixed Arekin's Sentinel picatinnies so that they're more printable and confluent with clam-shell designs for easy disassembly.

Added a 1.1 variant where less rails were present at the front and back, as you never use that part of your rails anyway. Should be faster to print and waste less filament.

Replaced everything with version 1.2.
Realized I forgot to level everything to build plate. [2019/3/31]