We don't need all the client-side routing part, we only need to be able
to selectively run certain JavaScript code depending on which site the
user is at the moment.
This implementation is not as advanced as the one from the `page`
package, or what is possible with the `url-pattern` package. But we
don't need any fancy URL matching at the moment, once we do we can
revisit and maybe introduce something like `url-pattern` instead of just
a crude `document.location.pathname === path`.