[MM-37933] Channel preference to auto-follow all threads in the channel (#21430)

* Add auto-follow feature
---------

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/platform/types/src/channels.ts

@@ -25,6 +25,7 @@ export type ChannelNotifyProps = {
     mark_unread: 'all' | 'mention';
     push: 'default' | 'all' | 'mention' | 'none';
     ignore_channel_mentions: 'default' | 'off' | 'on';
+    channel_auto_follow_threads: 'off' | 'on';
 };
 
 export type Channel = {