Currently the ability to reply in threads was added but not the ability to actually view threads so this doesn't currently make much sense to just have enabled int he main build.
Note: I want to cherrypick this so it's just the flag. I'll add a feature flag page to dev tools for master soon.
This MR deals with only sending threaded messages. Showing threads will turn up in a follow up. This allows you to start a new thread by clicking reply in thread to a normal message.
You can also do a threaded reply to a threaded message in the main timeline at the moment because those messages aren't shown in a separate thread timeline yet but will be in future.
