Make it easier to test the development client from a different device by removing
the need to customize the `SIDEBAR_APP_URL` and `PACKAGE_SERVER_HOSTNAME`
environment variables first.

Instead of hardcoding the client/h dev server hostname and scheme in the client's
boot script, support a very simple URL templating scheme where
references to `{current_host}` and `{current_scheme}` are replaced by
the corresponding elements of the boot script's URL.

This means that for example, if the embed script is accessed as
https://Roberts-MacBook-Pro.local:5000/embed.js then the boot script
will use asset URLs such as
https://Roberts-MacBook-Pro.local:3001/assets/sidebar.bundle.js. If
the boot script is accessed as http://localhost:5000/embed.js then
it will generate http://localhost:3001/assets/sidebar.bundle.js
instead.

A related change in h will remove the need to set the `CLIENT_URL` env
var and modify the OAuth client's redirect URL. At present, those steps
are still required.