Include the campaign property's Wikidata label

* Add the WikiGrok\WikibaseConnector#getLabelForProperty method, which
  returns the label for the property in the content language of the host
  wiki
* Modify WikiGrok\QuestionPicker#getQuestions to filter campaigns that
  don't have labels and to include the labels in its result
* Migrating the property ID from 'property' to 'propertyId'
* Incrementing FORMAT_VERSION

Change-Id: I97bfd010ac18b6a6ef8129086028c040df38f4fc
Merge "Include the campaign property's Wikidata label"