Turn on auto-vacuuming, which keeps file size down after deletes and prevents DB fragmentation

This can't be enabled on databases with existing tables, so for existing users we can either a) add a FAQ with instructions to recreate and let the advanced users do it and let everyone else have larger tables or b) add a migration step to dump to a new file and then replace the old with the new, which may be a risky and not all that easy proposition (given the lack of .dump) but might work...

The steps, by the way, with Firefox closed:

$ sqlite3 zotero.sqlite
sqlite> .out dump.sql
sqlite> .quit
$ sqlite3 db.sqlite
sqlite> PRAGMA auto_vacuum = 1;
sqlite> .read dump.sql
sqlite> .quit
$ mv db.sqlite zotero.sqlite

chrome/content/zotero/xpcom/schema.js

@@ -311,6 +311,9 @@ Zotero.Schema = new function(){
 	function _initializeSchema(){
 		Zotero.DB.beginTransaction();
 		try {
+			// Enable auto-vacuuming
+			Zotero.DB.query("PRAGMA auto_vacuum = 1");
+			
 			Zotero.DB.query(_getSchemaSQL('userdata'));
 			_updateDBVersion('userdata', _getSchemaSQLVersion('userdata'));