Allow embedding outputs from deployed apps

[josevalim]

You should be able to embed the output of a deployed app, so you can, for example, attach them to notion.so and other places.

To do so, we will generate a URL with a token. The token should be generated in function of the password ("" if there is no password) and contain a reference to the index of the output type. For example, the second graph in the cell. Changing the password will make the token invalid.

[josevalim]

Btw, you should be able to embed the whole app without problems and that's most likely the most robust way to share it that does not depend on specific location of outputs.

However, once we introduce #2109, then you should be able to share rows of a panel just as well.