Ensure xml_hash is not null and stop doing full-text search

[jnm]

• add data migration
• enforce uniqueness on xml_hash?
• get rid of this full-text search:
  

  kobocat/onadata/libs/utils/logger_tools.py

  Lines 165 to 168 in c7e9cf0

  	existing_instance = Instance.objects.filter(
  	Q(xml_hash=xml_hash) | Q(xml_hash=Instance.DEFAULT_XML_HASH, xml=xml),
  	xform__user=xform.user,
  	).first()