Use metric units by default

[louh]

Internally, units are stored as imperial units. Migrate to using metric units by default (including updating all data) for better worldwide use.

Project: https://github.com/streetmix/streetmix/projects/11

[louh]

Since there can only be two kinds of units - metric and imperial, always assume that when there's no units provided, use metric. Right now, internally, imperial is assigned a value of 1 and metric is assigned a value of 2. We can also change this so metric has a value of 0. This way, lack of value = metric, and the current imperial units constant can be changed to IS_IMPERIAL=1.

[louh]

Superceded by #2900