refs #4996 we have to add build a fully qualified URL of the content piece in...

refs #4996 we have to add build a fully qualified URL of the content piece in case we detect the URL of an image, video or audio, pdf, ... automatically. Otherwise we cannot display a preview in the UI and one would not know which URL was actually meant. Thinking about using //domain/path instead of http://domain/path as it would track different content pieces for http and https otherwise