Make WMS feeds more conventional

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ForWarn II WMS feeds use an unconventional MAPFILE query parameter in order to determine the correct mapfile to draw from on the backend. (In the past the system would overload the meaning of the standard WMS query param "LAYERS" in order to find the correct mapfile, so the mapfile had to be the same name as the id of the layer.)

This approach causes some services (like Terria) to be unable to ingest our WMS.

What we need instead is to use the standard WMS query parameter "MAP" to provide a path to a mapfile. (See this page for a roadmap to how this works securely.)