Also:
* _finalizeErase in Zotero.DataObject is now inheritable
* Call _initErase before starting a DB transaction
* removes Zotero.Libraries.add and Zotero.Libraries.remove (doesn't seem like this is used any more)
And add group.fromJSON(json, userID), which sets editable and
filesEditable properties based on the group JSON (libraryReading, role
lists, etc.) and the given user