Change interaction UI for adding users when creating streams

[timabbott]

Originally, this was part of #29403.

The biggest usability challenge with the stream creation panel right now is that adding users via the "Add" button is not intuitive in the stream creation context. We should:

• Remove the Add button. Entering a pill should immediately add users to the table below.
• Keep the pills in the box, and let it stretch to multiple lines to accommodate more pills.
• Explode groups into individual user pills when added, like we do in the DM compose box. In the future, we should have a different concept for adding a group, but right now we are in fact adding users as individuals. This is necessary for keeping the pills and the table below consistent.
• Removing users should be synced across the pills area and the table. So deleting a pill should delete the user from the table, and clicking "Remove" in the table should delete the pill.

[zulipbot]

Hello @zulip/server-onboarding, @zulip/server-streams members, this issue was labeled with the "area: onboarding", "area: stream settings" labels, so you may want to check it out!

[alya]

The current plan is to sequence this after the part that's still in #29403.