Merge attachments and update notes (#2336)

We follow a different merge procedure for each attachment type:

- For PDF attachments, compare by MD5. If no match, get the top 50 words
  in the attachment's text and hash those, then check again for a match.
  Update references to item keys in notes and annotations.
- For web (snapshot / link) attachments, compare by title and URL.
  Prefer a title + URL match but accept a title-only match.
- For other attachment types, keep all attachments from all items being
  merged.

Also:

- Move most merge tests from Duplicates to Items#merge(). It just doesn't
  make sense to worry about the UI in these.

chrome/content/zotero/xpcom/data/item.js

@@ -2916,7 +2916,7 @@ Zotero.Item.prototype.deleteAttachmentFile = Zotero.Promise.coroutine(function*
  * Return a file:/// URL path to files and snapshots
  */
 Zotero.Item.prototype.getLocalFileURL = function() {
-	if (!this.isAttachment) {
+	if (!this.isAttachment()) {
 		throw ("getLocalFileURL() can only be called on attachment items");
 	}