Add length helper functions

Converts geometry to geography and gives real lengths in metres.

Two strategies implemented: PostGIS and RGeo. Both give roughly the same (but not identical) results.
Add length (m) to bikeways.

During import, we now populate `length_m` fields on Bikeway and BikewaySegment.

Run `annotate` command.
Merge branch 'add-length-in-metres'