Allow drivers to set custom course tracking zoom level

[ericrwolfe]

If you try zooming out (two finger tap and drag up/down), the app immediately returns to the original zoom.

We should allow zooming out while staying fixed on followWithCourse, and keeping the zoom level fixed at the user’s new setting.

cc @danesfeder

[akitchen]

This is unlocked by gesture handling changes LocationLayerPlugin v0.5.0