Fix Keystone 500 battery holder

This merge request fixes #2602 (closed) Keystone 500 12mm coin cell battery holder.

Datasheet: http://www.keyelco.com/product-pdf.cfm?p=710

• dimensioning/positioning done in inches to match datasheet's primary dimensions
• moved pad 2 (negative terminal) to match datasheet location
• changed pad 1 shape from circular to rectangular pad
• re-drew component outline on fabrication layer to match datasheet dimensions
• added "+" polarization markings on fab and silkscreen layers
• re-drew silkscreen
• re-drew courtyard
• added "12mm" to description
• updated 3D model directory to KiCad 7: KICAD7_3DMODEL_DIR