Add a new pin

[Marco Pegoraro]

• paste a url and create instance, frontend is loading
• trigger backend process to resolve the details
• frontend stops loading, show the details, let edit title and content
• add and remove tags even while loading

The "loading" mechanism should be a client subscription to the newly created pin.
A piece of data "is_fetched" becomes true when the item is processed.

Options for server communications:

• Hasura trigger
• Backend subscription
• Postrges trigger that adds a queue task
• Postgres trigger that adds a message, backend subscribes to messages