restream service configurations

[JanKoppe]

The current mockup of the restream app assumes that there is just a "streamtarget".

We probably should offer some concept of streaming services, where a restream configuration targets different services (twtch, ytube, etc) and the users only have to insert their streamkeys. All the target servers, backup servers, maybe necessary transcodes for compatiblity should be automatically handled in the background.

Of course the minimum implementation of such a restream service is just a "Custom RTMP endpoint" service, where you can throw in a RTMP target, and we will push the stream without any changes.