FEATURE: introduces group channels (#24288)

Group channels will allow users to create channels with a name and invite people. It's possible to add people even after creation of the channel. Removing users is not yet possible but will be added in the near future.

Technically a group channel is `direct_message_channel` with a group attribute set to true on its direct message (chatable). This model might evolve in the future but offers much flexibility for now without having to rely on a complex migration.

The commit essentially consists of:
- a migration to set existing direct message channels with more than 2 users to a group
- a new message creator which allows to search, add members, and create groups
- a new `AddUsersToChannel` service
- a modified `SearchChatable` service

plugins/chat/spec/support/api/schemas/user_chat_channel_membership.json

@@ -15,7 +15,6 @@
     "muted": { "type": "boolean" },
     "desktop_notification_level": { "type": "string" },
     "mobile_notification_level": { "type": "string" },
-    "last_viewed_at": { "type": "string" },
     "following": { "type": "boolean" },
     "user": {
       "type": ["object", "null"],
@@ -24,7 +23,9 @@
         "id": { "type": "number" },
         "name": { "type": "string" },
         "avatar_template": { "type": "string" },
-        "username": { "type": "string" }
+        "username": { "type": "string" },
+        "can_chat": { "type": "boolean" },
+        "has_chat_enabled": { "type": "boolean" }
       }
     },
     "last_viewed_at": { "type": "datetime" }