Invite links for groupActivityOf sessions

Came up in discussion with @jbash in #1409

Currently, getting a multi-user session going with `groupActivityOf` is a two-step process:

1. Get everyone running the same code (identified by a hash or dhash).
2. Someone creates a session, and everyone else joins the session using the four-letter code.

This is more complex than it needs to be. Instead, what could happen is:

1. ONE player runs the program and creates a new session.
2. They get an invite link, which they send to everyone else, who uses it to run the same program AND automatically join the session.

Basically, the invite link should have both the program hash/dhash and the session code embedded into it.