mention: Allow silent mentioning system user groups.

Previously, we do not allow mentioning system user groups
at all. Now we want to use silent mention syntax for system
groups in the message sent when updating the posting permission
for a stream, so it is important to allowing silent mentioning
system groups at least. And there is no problem in allowing
silent mentions of system groups for all users.

We do not allow mentioning system groups as can_mention_group
for them is set to "Nobody" group.

zerver/lib/markdown/__init__.py

@@ -51,6 +51,7 @@ from zerver.lib.mention import (
     FullNameInfo,
     MentionBackend,
     MentionData,
+    get_user_group_mention_display_name,
 )
 from zerver.lib.outgoing_http import OutgoingSession
 from zerver.lib.subdomains import is_static_or_current_realm_url
@@ -1989,7 +1990,7 @@ class UserGroupMentionPattern(CompiledInlineProcessor):
 
                 if not silent:
                     self.zmd.zulip_rendering_result.mentions_user_group_ids.add(user_group.id)
-                name = user_group.name
+                name = get_user_group_mention_display_name(user_group)
                 user_group_id = str(user_group.id)
             else:
                 # Don't highlight @-mentions that don't refer to a valid user