Add a string->float maintenance script

A mobile app bug caused a lot of our geometry coordinates to get stored as strings instead of floats. The `float-coords.js` maintenance script updates those entries with float coordinates. It should only take a minute or so to update a few tens of thousands of errant entries, but it should be run during off hours, since we can't make much use of indexes.

Tested against a local mongo database and a remote db on mongolab.

/cc @hampelm