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/requests/chat_controller_spec.rb

@@ -611,6 +611,7 @@ RSpec.describe Chat::ChatController do
       expect(response.status).to eq(403)
 
       Chat::DirectMessageUser.create(user: user, direct_message: dm_channel.chatable)
+
       expect {
         post "/chat/drafts.json", params: { chat_channel_id: dm_channel.id, data: "{}" }
       }.to change { Chat::Draft.count }.by(1)