Require @-prefix for adding a user group to a stream

[alya]

It is possible to add a user group to a stream by typing in the name of the group.

To make this feature easier to use and explain, we should change it so that you have to type '@', followed by the name of the group. This will make user groups easier to find in the typeahead.

We recently merged 1b91545, which assumes this behavior.

[zulipbot]

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

[alya]

@m-e-l-u-h-a-n could you please prioritize working on this issue? CC @timabbott

[m-e-l-u-h-a-n]

Opened #18891 for this.

[alya]

Closing in favor of #22448.