resuming a shared chat says you are offline but you are not

[quozl]

When a shared chat instance is resumed, a message appears "Off-line: Share, or invite someone.", yet the other users see the chat in their neighbourhood view and the local chat briefly shows it is not shared, then quickly shared.

The inconsistency might be solved by one of these changes;

• all resumed instances must not share without new request from learner;
• a resumed chat instance in particular must not share without new request from learner; or,
• the check in Chat.__init__ must better detect resuming a shared instance; which probably doesn't happen right now because Activity.__init__ can return before sharing is completely set up.

Use case is to review a chat log without calling for participation by others.