Properly wire feature flag functionality in ConversationVisitor

Related to !111 (merged)?

• https://code.briarproject.org/briar/briar-desktop/-/blob/d1e668dec33d8f309a6f6d99bbc8b4af015475d2/src/main/kotlin/org/briarproject/briar/desktop/conversation/ConversationVisitor.kt#L90
• https://code.briarproject.org/briar/briar-desktop/-/blob/d1e668dec33d8f309a6f6d99bbc8b4af015475d2/src/main/kotlin/org/briarproject/briar/desktop/conversation/ConversationVisitor.kt#L134
• https://code.briarproject.org/briar/briar-desktop/-/blob/d1e668dec33d8f309a6f6d99bbc8b4af015475d2/src/main/kotlin/org/briarproject/briar/desktop/conversation/ConversationVisitor.kt#L178