Allow extended filtering on relationships

[merkys]

This PR addresses #437 by allowing fully-fledged filtering on relationships. Prior to it, filtering was allowed, but only on id and meta.description of relationships. Now all properties of related entries should in principle be allowed to filter on, to arbitrary depth (OPTIONAL).

This introduces a slight backwards-incompatibility by abandoning special treatment for description. Thus queries which previously matched/mismatched, but were perfectly legal, now will return HTTP 400 for all standard entry types except files where description property is defined since v1.2.0. I am not sure how severe this incompatibility is - if we want to avoid it, we will have to mandate the special handling of description by re-routing it to meta.description of a relationship instead of entry's property.

Fixes #437.