Load errors if aggregator turned off

Since the aggregator includes use aggregator_filter_xss() and that function is only present if aggregator.module is turned on, it is possible to have a case where a user's mySite treis to invoke aggregator.inc and we get a fatal error.

Two solutions:

- When running mysite_load_includes() for a user, check the load against the current active types.
- Wrap the use of aggregator_filter_xss in an if function exists check and use filter_xss() if it fails.

This problem is the reason why the MySite cron function sweeps through and de-activates dead items in a user's list.