Revise/redo page metadata (NL and EN)

[kintopp]

These are the summaries Google reproduces in search results. Most of the ones we have now are either placeholders (e.g. first sentence of the page) or otherwise don't fit. Best to leave them blank (Google will substitute its own summary or first sentence) until they can be re-written later.

[kintopp]

@Doppen I know what the meta_description metadata is and how it's used by Google. But how does Google use (if it does) the summary metadata?

[kintopp]

This also applies to meta_keyword