[FEATURE] Add sorting column for publications

[mschwemer]

The publication list is build on the columns month, day andyear. The date is build on each request for each found publication. Futhermore the publications are then sorted via a array usort.

It would be better to have a column "publication_time stamp" and use already for sorting in the database query.

What would be needed for this change:

• add the column
• provide a migrator for the existing records
• adapt the importer and migrate command to fill the additional column
• extend the data handler to update the column on record save in the backend