Add the shareable ID to a notebook's URL on the first time it is shared

[agriyakhetarpal]

This PR introduces the shareable URL as a URL parameter to the notebook URL when it first connects to the sharing service (on manual shares). As with other instances, we use window.history.replaceState to modify the URL without reloading the page. This guarantees that if an authored notebook is reloaded, the shareable link in the URL will still enable users to access a view-only version, from which they can create another copy and retain their work. To generate a new notebook, students will need manually remove the URL parameter themselves. As discussed, since the save interval is thirty seconds, there is a slight chance that changes made between these intervals might not be saved, but I would emphasise that we are already doing our best to prevent data loss through other methods (toast notifications, handling beforeUnload events, etc.).

Currently, this cannot be reliably tested until I complete #73, so I've skipped adding a test. However, I have tested this locally.

As discussed internally, this is slightly orthogonal to the sharing token being persisted using a cookie or the notebook content being saved to a cookie with an extremely short expiration time.

Closes #104.

[krassowski]

Thanks!