Add custom layers in frontend settings without having to edit the source code

[sgofferj]

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Is your feature request related to a problem? Please describe.
Maps comes with a limited selection of available layers which do not suit everybody. E.g. openstreetmap is useless in rural Finland because it doesn't cover it well. Adding new layers requires manual editing of mapLayers.js which is uncomfortable and collides with the signature checks.

Describe the solution you'd like
Please add the possibility for admins/users to add additional XYZ/WMS/WFS map layers through frontend settings.

Describe alternatives you've considered
There is no alternative

Additional context
n/a