Add `webmap-import` script to import source layers.

There is still a few things to do (such as reprojection and geometry
changes) but it's mostly working.

We roll out our own ogr2ogr/GDALVectorTranslate()-like function version
because GDALVectorTranslate() insists in calling StartTransaction()
https://github.com/OSGeo/gdal/issues/3403 while we want a single
transaction for the entire desination layer, including truncation,
source imports, and metadata changes.

Surprisingly our version is not much slower than the C++ one.  Importing
the 157446 (of 667034) features from sksUtfordAvverk-2000-2015.shp takes
14.3s while

	ogr2ogr -f PostgreSQL \
	    -doo ACTIVE_SCHEMA=postgis \
	    --config PG_USE_COPY YES \
	    --config OGR_TRUNCATE YES \
	    -append \
	    -fieldmap "0,-1,-1,-1,-1,1,2,3,4,5,6,7,8,9,10,11,12,13" \
	    -nlt MULTIPOLYGON -nlt PROMOTE_TO_MULTI \
	    -gt unlimited \
	    -spat 110720 6927136 1159296 7975712 \
	    -nln "sks:UtfordAvverk" \
	    PG:"dbname='webmap' user='webmap_import'" \
	    /tmp/x/sksUtfordAvverk-2000-2015.shp \
	    sksUtfordAvverk-2000-2015

takes 14s.

Merely opening /tmp/x/sksUtfordAvverk-2000-2015.shp and looping through
its (extent-filtered) features results in a runtime of 4.3s.

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