It is now possible to group locations.
Each location can have arbitrary children.
That can be used for editorial structuring.
Filtering for a location will always find all dates where the location
or one of the child locations is assigned.
One use case can be to group imported locations and provide a grouped
location for filtering in frontend.
Relates: #11233
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
