Allow disabling media_player volume slider in entities card

[zotanmew]

When using the media_player entity in an entities card, the volume slider shows up if supported. I traced this back to #4743, where the possibility of making it optional is briefly discussed but seemingly never implemented. I would like to hide the volume slider for visual consistency with the rest of the entities card, since I barely ever control the volume with HA. Even then, the more info panel has a slider. I therefore ask that you reconsider not making this option configurable.

This is how it looks right now:

This is how I would expect it to look with volume control disabled:

Thank you, ~zotan