[Meta] Roadmap for 2.0

This idea of this issue is to have a conversation to define a roadmap for the module. We already have some ideas, don't hesitate to comment to tell us what feature seems very needed or express other new needs and ideas.

# Must-have

1. Ready for Drupal 9
2. Clean and increase the stability of the code
3. #3086071: [Meta] Remove theme dependency →
4. Implement tests
5. Blocking system
6. The inbox is not updated when creating new threads

# Should-have

• Service worker with push notification
• Add people to a thread
• Default page should be the list of the conversations
• ✨ Realname support Needs review
• Rules integration
• Schedule thread/message creation

# Could-have

• Cleaning old messages

Please avoid to create the associated issues, I'll do it myself steps by steps in order to manage the issue queue more easily. Don't add small bugs, those ones should be fixed at anytime, we are more talking about general issues and features.

# Planned releases

• #3041652: [Meta] - Beta 16 → Locked
• #3091853: [Meta] - Beta 17 →

When a release is locked, only critical issues can be added.