Improve HTTPS cert validation for appstore

[georgehrke]

make code use https interface of appstore and properly validate it's certificate

cc @karlitschek

[georgehrke]

related to #8808

[karlitschek]

yep. will do the next few days

[georgehrke]

@LukasReschke could you do me the favor and evaluate if it's enough to make this https or if there is a need to strengthen Util::getUrlContent()

[PVince81]

Setting to OC 7 CE

[georgehrke]

ping @LukasReschke