https://ogp.me/ is used by some platforms, especially social media, to
generate previews of shared links.
The extension now delivers default open-graph tags for better user
experience.
Twitter uses its own way which is also supported.
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
