Show URL of REST API

[jl-wynen]

Users of the REST API, e.g. through Scitacean or PySciCat, need to know the address of the API. At ESS this is simply the address of the frontend plus api/v3. But IIRC, this is not the case everywhere. See also https://github.com/SciCatProject/scitacean/issues/17

In any case, it would be useful to list the URL somewhere. I'm not sure where, though. Maybe in the user settings, as Max suggested alongside the token, or on the help page would work.