Rooms can show or hide any connection messages :alien:

Reference: [Issue #695](https://github.com/davidfowl/JabbR/issues/695)
 
- Rooms have an IsConnectionDisplayed boolean property.
- New ToggleConnectionMessage admin/room owner command.
- Connection messages are displayed or muted, depending on the room IsConnectionMessageDisplayed state.
- Database migration included to update the ChatRooms table.