Create sharable links

[danfishgold]

Allow sharing an annotated code as a link. People who open the link will see the code along with the sharer's annotations.
Any annotations they make won't update the ones on the link (so if A shares and B opens and edits, when C opens the link they won't see B's edits) but you can create a new link with your new annotations.

Some resources around TTL on Firebase:

• TTL in Firestore
• Deleting documents from Firestore
• TTL in Firebase Storage