[Feature] Allow Admins/Mods to seek the video

[bgiesing]

Mods and Admins should be allowed to seek position in a video that will be synced up to everybody else. This is good for videos that have a long intro that could be skipped and could also act as a potential fix for #98 (as you could just seek back to the point the server disconnected).

[DerpgonCz]

I'd say a feature for users to set up where a song starts / ends would be nifty aswell. But that would have to wait until #43