Handle symmetric relations (Solr case)

Some relations such as "knows" or "is related to" are symmetric, as opposed to others such as "has author" which are not. This issue is about making sure that, once the switch to Solr is implemented for adding and displaying existing statements, it is possible to view, on an existing pages, both the outgoing statements (e.g. on page A : A is related to B), and incoming ones (e.g. C is related to A).