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/db/migrate/20231006160650_add_group_field_to_direct_message_channels.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddGroupFieldToDirectMessageChannels < ActiveRecord::Migration[7.0]
+  def change
+    add_column :direct_message_channels, :group, :boolean, default: false, null: false
+  end
+end

plugins/chat/db/migrate/20231006161051_set_multiusers_direct_message_channels_as_group.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class SetMultiusersDirectMessageChannelsAsGroup < ActiveRecord::Migration[7.0]
+  def change
+    execute <<-SQL
+      UPDATE direct_message_channels
+      SET "group" = true
+      WHERE id IN (
+        SELECT direct_message_channel_id
+        FROM direct_message_users
+        GROUP BY direct_message_channel_id
+        HAVING COUNT(user_id) > 2
+      );
+    SQL
+  end
+end