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
There might be access to TSFE get_cache_timeout() prior rendering
events.
TYPO3 has a cache for timeout and won't re calculate again.
We therefore need to clear the cache if timeout would change.
That will lead to inconsistent cache information throughout a single
request. But the final cache timeout of the page will be correct. Other
parts might be longer, which probably is fine until they relate to the
events.
Relates: #10500
