Add units toggle

[quincylvania]

Mostly we care about meters/feet/miles for things like distance, height, and elevation. But other units will certainly pop up too.

There are a few styles to consider:

• Metric
• UK: miles for distance
• US: miles and feet as far as the eye can see
• Auto: detect based on browser locale
• Local: dynamic based on the units used in the country where each feature is located
• Dual: show both metric and imperial units at once