Enable stepless adjustment of the volume control slider

Previously, the volume would be adjusted in 2% steps, which is needlessly imprecise and also makes for poor UX. This change makes the volume slider smoothly adjustable.

CC @phw : Are there any hidden gotchas with stepless adjustment with regards to logarithmic volume scaling?

TODOs:

• Rescale volume slider to 0..100 to keep keyboard input functional

This Merge Request includes:

• Tests
• A changelog fragment (cf https://docs.funkwhale.audio/contributing.html#changelog-management)