mine first image as image for twitter card (#1772)
mine first image as image for twitter card (#1772)

* mine first image as image for twitter card

* more restrictive regex

switchinnovations/pxt,switch-education/pxt,playi/pxt,switchinnovations/pxt,switchinnovations/pxt,playi/pxt,playi/pxt,Microsoft/pxt,switch-education/pxt,switch-education/pxt,playi/pxt,switch-education/pxt,Microsoft/pxt,switchinnovations/pxt,Microsoft/pxt,switch-education/pxt,Microsoft/pxt,Microsoft/pxt