UX: Admin setting page consistency - Group permissions (#30528) (#30578)

* UX: Admin setting page consistency - Group permissions (#30528)

Followup c2282439b32d879a73217eec62449f042914d7d0 and
21470e4afd49127fc6281a5bde6357e86b11a4ae

Adds a new "Group permissions" page and group_permissions site
setting area, which shows every single group-based site setting
in the app and core plugins.

Also adds a "trust_levels" area to show on the already moved
admin "Trust level" settings page, since that previously was
showing a mix of trust level settings and allowed group settings.

* DEV: Review fixes

plugins/chat/config/settings.yml

@@ -12,6 +12,7 @@ chat:
     mandatory_values: "1|2" # @admins, @moderators
     allow_any: false
     refresh: true
+    area: "group_permissions"
   chat_threads_enabled:
     client: true
     default: false
@@ -101,13 +102,14 @@ chat:
     allow_any: false
     refresh: true
     validator: "Chat::DirectMessageEnabledGroupsValidator"
+    area: "group_permissions"
   chat_message_flag_allowed_groups:
     default: "1|2|11" # @admins, @moderators, @trust_level_1
     mandatory_values: "1|2" # @admins, @moderators
     type: group_list
     allow_any: false
     refresh: true
-    area: "flags"
+    area: "flags|group_permissions"
   max_mentions_per_chat_message:
     client: true
     type: integer