Provide return link to booking page from experiment UIs when session has ended #20

As suggested by @ImogenWren , if we could provide a link back to the booking page in the experiment UI when a session has ended, that would help users carry on with more experiments if they want. We'd need to provide this via the config file or a query param, because it might be too brittle to hardcode it into the experiment UI in case that is distributed between multple tenants (each at the same baseURL, so far as the UI is concerned).