Add stream setting for who can unsubscribe

[alya]

For some organizations, it would be useful to restrict permissions for unsubscribing from certain streams. For example, this can help ensure that users don't miss important announcements, as discussed in this CZO thread.

We should add a setting for which groups can create unsubscribe from a stream after the relevant infrastructure in #19525 is completed, with "everyone" as the default setting.

[zulipbot]

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