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/test/javascripts/components/chat-composer-placeholder-test.js

@@ -36,6 +36,7 @@ module(
       this.channel = ChatChannel.create({
         chatable_type: "DirectMessage",
         chatable: {
+          group: true,
           users: [
             { name: "Tomtom" },
             { name: "Steaky" },