Scope of this task is roughly:
- New MessageGroup class
- Group loader
- Using hooks and metadata tracking to register groups and track which revision has been seen
Leaving for later:
- Synchronization
Additionally, this feature should probably be off by default until it is fully implemented.
Expected outcome
User can create a page with MessageBundle content format. If the content is valid, there is a message group on Special:Translate (but translation will not work fully).