GeoJSON output should try to set the "crs" property if possible

[jng]

When outputting GeoJSON, we should already know about the coordinate system of the feature data we're writing already, so as a result we should be trying to also write out the "crs" property so the SRS GeoJSON can be self-identified.

An example of what to write in this property: ​https://gis.stackexchange.com/questions/46881/ogr2ogr-from-postgres-to-geojson-missing-crs-key?rq=1

[jng]

In 10021:

When outputting GeoJSON from our mapagent, always try to write the "crs" property if the SRS of the feature source we're writing the GeoJSON from has an EPSG code representation

Also fix application/json mime type not being set for GeoJSON representations of WFS GetFeatures

Also make sure that the server implementation of MgFeatureService::GetWfsReader() applies coordsys transforms if one is constructed in the MgWfsQueryAdapter. This means that GeoJSON representations of WFS GetFeatures are properly transformed like the other supported formats.

Fixes #2809
Fixes #2857
Fixes #2856