add dog-off-leash icons

Because there's no single facility record useful across-the-board
for identifying a dog-off-leash area, numerous additions have been
made to support dog-off-leash icons.  It will also be useful for
future features, or further distilling/categorising existing
facilities of interest.

Outline of changes:

1) Add 'park_feature' table.

2) Update import program to calculate the centroid of all facilities
   associated with a DOG OFF LEASH NODE for a single park and insert
   a single record into 'park_feature'.

3) Update the 'Park' record type to have a '[Feature]' field.

4) In the UI code, map park features to icons.  Unlike facilities,
   there is no indirection for features - the featureId is used as
   icon key.