This button was red, which is appropriate when modifying an existing stream,
since that's a potentially disruptive action, but not appropriate in the context
of previewing subscribers for a new stream being created.
Fixes: #21863.
The most notable change here is that when you are adding
subscribers to a stream as part of creating the stream,
you can now use the same essential pill-based UI for
adding users as we do when you edit subscribers for an
existing stream.
We don't try to exactly mimic the edit-stream UI or
implementation, since when you are adding subscribers
during create-stream, we are just updating a list in
memory, whereas in the edit-stream UI, we immediately
send info to the server.
Fixes#20499
Harshit Agarwar
Heidi Ahlberg
Steve Howell