Change open answer questions

* store distinct open answers per question in lookup_answer_label
* lookup_answer_label: answer_label is now 1024 (open answers longer
than 1024 are truncated)
* fact_answer:
	* if answer is numeric: store answer and answer_label_key
	* if answer is string: store empty answer and answer_label_key

* remove comments to database fields (it made keys nullable)