rearlight.scad: first printed version

BUGS:
- make rear spring a bit stronger
- wall_thickness could probably be lowered to 1.5mm
- fraen_h should be lowered about 0.5-1mm
- side barriers are too high
- clip_angle is too big - about 18 degrees should be enough

TO CHECK:
- bolt sizes
- side LED hole size
- cable hole

TODO:
we use elliptical beam lens, so for two lenses it could make sense
to not have the clip and lens axis parallel, but instead to make
it shine more to the side.