Add new config option allowVisibleSource

[roman-yatsenko]

Add new config option allowVisibleSource to the project. The default value for this option is true. The false value will block Edit and Both editor modes for users that have no rights to edit a note.

Pull request adds:

• new config option allowVisibleSource with default value true;
• new environment variable CMD_VISIBLE_SOURCE;
• disable Edit and Both modes for users without edit note permission;
• redirect form Edit and Both modes to View mode by direct URL with mode parameter.