publication extensions: have a special column for aggregated publication info

[dbu]

Group all info of the 3 publication admin extensions in one column: Published, XYZ until XZY | Published, XYZ | Published, until XZY. Use a color code to reflect the current state.

This should be an optional feature, some might prefer to have separate columns for ordering and filtering.

That would probably work like this: https://github.com/symfony-cmf/CoreBundle/blob/master/Admin/Extension/TranslatableExtension.php#L50

Suggestion from https://blog.liip.ch/archive/2014/12/11/improving-the-user-experience-of-the-symfony-cmf-backend.html : We could use an icon for the column heading of publishable to save some space.