Add code to auto-invite all users in a hunt to each new puzzle channel

Add code to auto-invite all users in a hunt to each new puzzle channel

We had originally written the code this way, long ago, but disabled it
(prior to MH 2021) when some people found it intrusive to be invited
to every channel, (rather than selecting the puzzles they were
interested in participating in). And after disabling it we also
deleted the code entirely in commit
ef8d8f359be149adbbdbeda88ec4b6bbbebf928e

Prior to MH 2022 we had talked about adding a user preference since
some people would like to be auto-invited. But then we got a lot less
code written than we'd wanted to before MH 2022.

So we decided to add the auto-join feature back, (so that people could
see channels that have huddle icons for easily determining puzzles
that have active activity).