Chat

Simple first version:
- Message like remark but always between applying users and handling users (public to all)
- Open/close chat per application
- Chat is only possible if the handler has first opened the chat (off by default). E.g. handler has a question, they can open the chat and ask the applicant. Some REMS instances would use email only.
- One action button and form with open/close and message functions.
- New email when message
- New commands/events (open, close, message)
- Available for all, no feature flag needed (can use disable commands to hide)
- All tests, including browser
- Available as remark or not available after "end state"?

Possibilities to extend:
- Workflow option to enable chat by default
- Track which events / chat messages the user has seen and always show the new ones (in the event log or else)
- Separate chat messages from the event log to a "chat thread".
- Messaging between any users (of the application, e.g. from handler to decider, not visible to others)

See also #2058