Use logarithmic scale for volume slider.

Fix #1222 (closed)

This Merge Request includes:

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

This uses a logarithmic scale for the volume slider, resulting in a more linear experience of volume differences when changing volume.

For good background on this please see https://www.dr-lex.be/info-stuff/volumecontrols.html