Use https instead of // everywhere by default

[mramato]

Traditionally, using // was standard practice because it would use the protocol of the existing page, i.e. http on http pages, https on https pages. With the pending transition to https everywhere, it makes sense for us to replace // everywhere we currently use it with https instead.

Furthermore, using // in offline apps (such as cordova or webviews) causes // to become file:// which means Cesium is broken by default in those situations. This will save developers writing those apps the initial headaches they've had in the past.