In order to achieve better relevancy, especially in item suggesters, it would be nice to index certain statements for select properties, such as `P31` (instance-of), etc.
This would allow to boost/de-boost certain classes (like disambig pages or templates) when searching for items, and get more relevant results.
Current plan:
* Add configuration that allows to specify which properties to index (by P-id)
* The index mapping creates a keyword field for each of these
* The value is indexed as single string, for entities that would be Q-id or P-id, for quantities - main value. TBD: what to do with complex types like coordinates.
* Qualifiers, references, ranks, etc. will be ignored for now
** Maybe with possible exception of excluding deprecated rank in next iteration?
* Develop a way to boost/de-boost certain things using this information (will be in a separate task)