BUG Prevent session-interface mismatch.

Disables transparent subsite switch on AJAX requests.

Makes sure the subsite is appropriately set up when opening up the CMS
with a link to subsited object.
Merge pull request #120 from mateusz/subsite-model-switch

BUG Prevent session-interface mismatch.
SubsiteDomain don't call decorators updateCMSFields

DataExtension#updateCMSFields() is only called if the DataObject::getCMSFields() first calls parent::getCMSFields()