Preference to adjust zoom rate

[0HyperCube]

In the preferences dialogue, an option should be made available for changing the zoom rate. As suggested by @j-jasz in #2407.

Currently it is set as a hardcoded VIEWPORT_ZOOM_WHEEL_RATE. The default value is 0.005.

You may wish to use a log scale (?).

[Keavon]

This would also be useful because we currently pick a rate that's a decent middle ground between what's useful for scrolling and what's useful for pinching. It's a bit too fast for scrolling and a bit too slow for pinching. But browser APIs don't let us distinguish the two. This would let users pick the rate that better suits their input device.