Relations overhaul (requires new DB upgrade from 4.0)

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.

test/tests/relationsTest.js

@@ -0,0 +1,24 @@
+"use strict";
+
+describe("Zotero.Relations", function () {
+	describe("#getByPredicateAndObject()", function () {
+		it("should return items matching predicate and object", function* () {
+			var item = createUnsavedDataObject('item');
+			item.setRelations({
+				"dc:relation": [
+					"http://zotero.org/users/1/items/SHREREMS"
+				],
+				"owl:sameAs": [
+					"http://zotero.org/groups/1/items/SRRMGSRM",
+					"http://zotero.org/groups/1/items/GSMRRSSM"
+				]
+			})
+			yield item.saveTx();
+			var objects = yield Zotero.Relations.getByPredicateAndObject(
+				'item', 'owl:sameAs', 'http://zotero.org/groups/1/items/SRRMGSRM'
+			);
+			assert.lengthOf(objects, 1);
+			assert.equal(objects[0], item);
+		})
+	})
+})