ChatView: use explicit username in append_line()

[slook]

• Added: Clickable roomname tags to close Public chat room feed: Don't notify on new messages + clickable room names #2482
• Changed: Prepend chat logs into ChatView widgets upon tab activation rather than during application launch
• Fixed: Part of the timestamp was incorrectly tagged if the username is a short numeric that's also in timestamp
• Fixed: New room wall message was appended at the end instead of being prepended to start upon text entry
• Avoids the use of find() when inserting text for online chat messages, it is only needed when reading old log file lines
• Reduces counting the number of characters while inserting text unless doing so is really necessary

• Reverts msg.formatted_message as it is not part of the protocol, this direct tag approach doesn't require it anyway.

The idea of tagging lines in batches is for a more efficient on-demand log file readback mechanism. Not only is application startup time much improved, but there is also scope for reading back old messages from the log file on demand, even after new chat message lines have already been appended.

Reference draft PR #2540