We have some "inverse" properties but most properties don't have an inverse property.
Instead of creating more inverse properties make it so it is as easy to find statements for which the current item is the object as it currently is to find statements for which the current item is the subject. That way we won't need inverse properties.
Subtasks:
* "What statements link here" special page
* UI option at the bottom of the statements on an item page to display the statements that have the current item as their object as well as the currently displayed list of statements that have the current item as their subject.
* Wikibase-client function to access statements for which the current item is the object as easily as the client can currently access statements for which the current item is the subject
* Deletion of one of each pair of inverse properties (not needed now)