Strip path component from IM rest url
before passing to js-sdk. We continue to use the full URL for the
calls done by matrix-react-sdk, but the standard terms API called
by the js-sdk lives on the standard _matrix path. This means we
don't support running IMs on a non-root path, but it's the only
realistic way of transitioning to _matrix paths since configs in
the wild contain bits of the API path.
Once we've fully transitioned to _matrix URLs, we can give people
a grace period to update their configs, then use the rest url as
a regular base url.
