Add a cron job to remove any old enough soft-deleted chapters

[ob-ivan]

A followup to #5.

• Replace isDeleted property with nullable deleteTimestamp. NULL is interpreted as "not deleted".
• Extract portions of ComicsController.purge to ComicsService.
• Add a command to the console application.
• Test the command. Add a dry run option.
• Add a cron job on the server and update the installation instructions.

[ob-ivan]

Replace isDeleted property with nullable deleteDate. NULL is interpreted as "not deleted".

I need to update the repository methods that query the table with conditions on the isDeleted column.