Force inclusion of extra tags when Nominatim internally depends on them

[lonvia]

There are a couple of tags in the 'extratags' column of a place on which Nominatim depends internally: place, wikipedia, wikidata, capital. When writing a custom style, you had to manually ensure that these tags are included. That can lead to bad surprises.

This PR changes lua processing, so that all tags that Nominatim depends on internally are always included independent of what the style says. The line

flex.add_for_extratags('required')

is no longer required and will in fact throw an error. If you have this in your custom style, just delete the line.