Fix SiteComment AjaxServer comment saving.

This was updated to call SiteComment's internal cache clearing back in r76399 / 60a314ea38c8956878fd840c6a8329cfde00b93e but marking comments as spam/not spam wasn't correctly updated. Looks like we never noticed this as most spam was caused by Akismit. Calls to mark spam without this just hang.

I also took the liberty of updating postSave() to always call clearCache() to simplfy our calls, and to make sure the cache is cleared in more scenarios.