A new class is added which will add meta tags for dates and events.
The class has an interface which allows it to be replaced via DI to
alter behaviour.
Refactor import regarding data handler. We now also need to add a new
column "keywords". We use the new DataHandler approach.
But that approach only covered relations so far. We therefore refactor
that area to be more generic and use that one for new keywords column.
Relates: #10642
We migrated the part of the import to use DataHandler.
We didn't invest too much time as budgets are low.
Still the bugs are covered with tests and fixed.
Relates: #10782
