Full release of Threaded conversations

We've reached support on all major Gitter experiences except for IRC.

This issue is concerned with two main subjects:

• Removing the feature toggle code. Making threaded conversations available in every room.
• Preparing comms to go out as a part of the release

Removing the feature toggle code

1. Create an MR that removes code from https://gitlab.com/gitlab-org/gitter/webapp/-/merge_requests/1622 and https://gitlab.com/gitlab-org/gitter/webapp/-/merge_requests/1620
   • we have to decide whether to leave the generic "room based feature toggle" mechanism in our codebase or remove it completely and potentially re-introduce it if needed
2. Optional: Cleaning the mongo DB - removing threaded conversations toggle from room metadata

Communication to Gitter users

• Finding out what is the process to use our MailChimp mailing list?
• Crafting an email message and get it reviewed.
• Send out a campaign once the feature toggle has been removed