This allows Zotero.Relations.getByPredicateAndObject()/getByObject() and
Zotero.Item::getLinkedItem()/Zotero.Collection::getLinkedCollection() to
be synchronous, which is necessary for word processor integration.
Also adds Zotero.DataObjects::getLoaded(), which returns an array of all
loaded objects of the given type. This is useful for selective
reloading, for example with item.reload(['relations'], true).
Relations are now properties of collections and items rather than
first-class objects, stored in separate collectionRelations and
itemRelations tables with ids for subjects, with foreign keys to the
associated data objects.
Related items now use dc:relation relations rather than a separate table
(among other reasons, because API syncing won't necessarily sync both
items at the same time, so they can't be stored by id).
The UI assigns related-item relations bidirectionally, and checks for
related-item and linked-object relations are done unidirectionally by
default.
dc:isReplacedBy is now dc:replaces, so that the subject is an existing
object, and the predicate is now named
Zotero.Attachments.replacedItemPredicate.
Some additional work is still needed, notably around following
replaced-item relations, and migration needs to be tested more fully,
but this seems to mostly work.
