Add possibility to use a preferred streaming server

[ssapalski]

Some people might want to use a fixed streaming server to mitigate streaming issues in their location.
Currently there are four premium streaming servers: prem1, prem2, prem3, prem4.

It shall be possible to define the preferred streaming server which will be used if it appears in the playlist of a certain channel. If the server doesn't appear, a random server from the playlist will be chosen!