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/config/routes.rb

@@ -18,6 +18,7 @@ Chat::Engine.routes.draw do
     post "/channels/:channel_id/invites" => "channels_invites#create"
     post "/channels/:channel_id/archives" => "channels_archives#create"
     get "/channels/:channel_id/memberships" => "channels_memberships#index"
+    post "/channels/:channel_id/memberships" => "channels_memberships#create"
     delete "/channels/:channel_id/memberships/me" => "channels_current_user_membership#destroy"
     post "/channels/:channel_id/memberships/me" => "channels_current_user_membership#create"
     put "/channels/:channel_id/notifications-settings/me" =>