Restrict wildcard mentions

Organization administrators can set a policy for which users are allowed to use wildcard mentions in large streams (defined for this purpose as streams with more than 15 subscribers).

Zulip allows anyone to use wildcard mentions in streams with at most 15 subscribers. The default allows only organization administrators to use wildcard mentions in large streams.

Users permitted to use wildcard mentions by the organization's policy are warned that wildcard mentions will result in all subscribers receiving email and mobile push notifications.