[web server] #1642893 issue a 403 instead of a 500 on Unauthorized exception To do so, we allow an arbitrary code to be passed to ``error_handler``. Default error code is still 500.

[css] repair selected slice distinctiveness (with a nice, discrete bg color)

[view] ability to define a column header through custom label_column_<colindex> method, useful for table view to customize a specific column without specifying others