vibration/weight enhancements

This is (approximately) just before the 2018 antenna capstone started working on OreSat.

• This keeps the same form factor as previous versions.
• It uses far fewer fasteners to attach the +/-Y frames to the -X frame (total of 4).
• It avoids (in all cases) putting fasteners on the same slot that a solar panel or backplane screw hole sits on. Thus, all holes (except for the counter-bores) may be drilled as through holes.
• The +/-Y frames also now have slots cut out to reduce mass.
• The slots for the cards no longer create 0.4 mm thin features. (It's now 1 mm.)
• The +X frame now features hold-down beams, which prevent the +X sides of the cards from vibrating by holding them with slots similar to those on the +/-Y frames.