[feature] Placeholder for redacted messages

[Chirunos]

No message deletion placeholders can be confusing due to lack of context when reading a conversation or when receiving a notification for a message and later noticing it isn't there anymore when you look for it, especially for people coming from clients that support the feature.

For those who prefer an uncluttered chat history, the feature may be toggled on or off (opt-in by default?) in settings.