Deliver different "unknown_img.png" according content type

[henryk86]

Currently all unknown content is represented as the question mark icon. This should be extended:

unknown tags with no source set, source set to a taf or undefined, live = false: status quo:

New: tags with a stream as source set ("http...."), live = yes:

Used image:

(combination of https://www.flaticon.com/free-icon/ask_1923795 (current image) and https://www.flaticon.com/free-icon/instagram-live_5825570)