Fixes #1543

The unread message counter overlapped the "leave room" button, but if
a user clicked on the counter, the "leave room" button was still active,
causing users to accidentially leave the room.

Prevent this by hiding the leave buttons if unread messages are shown.

Allows better matches on markdown formatted urls

* Client-side showUserInfo is no longer cleared
* Server-side now handles empty messages
* Added error message if username or message are empty

Added check on username according to app settings for name validation.