Speed limits with missing units

[1ec5]

There should be a validator for maxspeed, maxspeed:advisory, and minspeed tags expressed in miles per hour without the required mph suffix. These values are interpreted by data consumers as kilometers per hour, so maxspeed=65 means 65 km/h or 40 mph, not 65 mph.

This query shows 209 ways in Ohio alone that have speed limits in miles per hour without specifying the unit. ito! Map also shows potentially mistagged speed limits.

/ref #266 /cc @maning