External xtal pads added, USB OTG connectivity

Additions:
- added 3.2 mm x 2.5 mm xtal pads on the back, connected with PL6/PL7
- reduced OHW logo size to 3 mm
- added optional connectivity from the USB OTG pin through PL1

Other minor fixes:
- fixed some minor connectivity issue showing in DRC due to Kicad bugs
- fix board to allow requirements to allow submission to OSH Park:
increase via sizes to 23 mils minimum, increase copper interval to board
edges to 15 mil