UI: Change icons to svg

This changes all of the icons from png to svg. With svgs, scaling is
unlimited. With the svgs, the issue of the @ symbols with the Windows
updater is no longer an issue.

I changed the colors of the icons to a light gray (#d2d2d2), in the
dark themes, because I thought they looked better with this color.

The mute, unmute, plus, minus, up, down and expand icons are from the
Feather Icons set. https://feathericons.com/

The visibility icon is from the Octicons set. https://octicons.github.com/

The locked and unlocked icons are from the Open Iconic set.
https://github.com/iconic/open-iconic