Phase 1: Delete all private messages in a conversation

• Add ConversationClient method for getting a set of message IDs to delete
• Add real implementation for MessagingManager, no-op implementations for other clients
• Add menu item for triggering deletion
• Add dialog for explaining that invitation/introduction messages can't be deleted
• Show dialog if not all message IDs in conversation were returned by clients for deletion

Estimated work, including review and testing: 1 week

Phase 2: Delete all completed invitation/introduction sessions in a conversation

• Get message metadata for contact group, map all messages to their sessions
• Look up session states and message statuses
• Exclude messages belonging to open sessions
• Exclude messages belonging to sessions with sent-but-not-acked messages
• Return IDs of remaining messages
• Update dialog to explain that ongoing invitations/introductions can't be deleted
• Show dialog if not all message IDs in conversation were returned by clients for deletion

Estimated work, including review and testing: 3 weeks

Phase 3: Multi-select messages to delete

• Add multi-selection to conversation view
• Intersect selected message IDs with those returned by clients, delete the intersection
• Show dialog if not all selected message IDs were returned by clients for deletion

Estimated work, including review and testing: 1 week