WFSSearchChannel: get region from a property

[jampukka]

Allow feature property to be used for the value of SearchResultItem.region as discussed in oskariorg/oskari-docs#280.

Stored in database table oskari_wfs_search_channels inside column config:

{
   ... rest of the config ...,
  "region-property": "foobar",
}

Example SQL for adding the value to existing config value (there's no UI for this yet)
UPDATE oskari_wfs_search_channels SET config = (config::jsonb || '{"region-property": "my_prop"}'::jsonb) where id = :id

[ZakarFin]

Nice 👍