Change date format

We currently only take the first date of a range as the date in Drupal and show it as a year. This leads to misunderstandings for dates with century accuracy.

We could implement the full ismi-date data model in Drupal but this would be a lot of work (first try in ismi-date-field branch).

We could use the ismi-date data model in the migration import to create a simple text representation, that is sortable and use it in place of the current datetime fields: 900-999 -> "09XX".