Add column-<field_name> classes to the admin list

This simplifies CSS styling to set column widths.
Merge pull request #834 from vdboor/issue_19906_list_classes

Added column-<field_name> classes to the admin list
Fixed python 3 support. Refs #17320
Fixes #6412 - More details about template loader.

Report more details about template files in loader postmortem.