Remove 'mount' config option #493

Currently, the --mount solid server config option allows for serving Solid from a "subpath" url. For example, if solid-server normally serves requests on https://example.com/, if you pass it --mount solid, then it would serve requests on https://example.com/solid/.

This is separate from the --root config parameter: root has to do with a directory on the server, and mount has to do with a url subpath.

Proposal: remove the --mount option from solid-server.

Removing this option would significantly simplify internal URL-calculating code for various API mount points. In multi-user (idp) mode, the mount parameter's behavior is undefined/un-implemented anyway. And in single-user mode, which url path a server listens on is a concern for whatever web server (Nginx, Apache, etc) is serving as a front end to Solid.