Only serve tagprofile HTML if we aren't POSTing via ajax

This fixes an issue where POSTing the selftag form in the
profile_block sidebar via AJAX would generate an XML response
containing both the content from doPost() and showPage(),
resulting in invalid XML.

These changes make it so that if we're POSTing via AJAX,
we serve content from doPost(), otherwise we serve showPage()
but never both.