Add a "Volume Lock" setting to zones/groups in the player page

[SteveMicroNova]

Add a "Volume Lock" setting so that individual zones or groups aren't effected by the volume bars that would effect all zones in the group/stream

The intended usecase for this would be so that if a zone is part of a group, or if someone uses the basic volume slider on the source, and individual zone / zones that are locked won't have their volume effected

I specifically envision this for home use where someone wants to tune in to something playing on a source that others are in on, but don't want their ears blown out if the person who "owns" the stream wants to turn up their volume using the main volume bar

[SteveMicroNova]

Note from Lincoln: remember to deactivate the mute button while the volume is locked, not just the volume slider

[rtertiaer]

I don't think we should do this.