Apply PST to the correct page

The "Preview page with this template" was applying the PST to the
content loaded from the specified page (which shouldn't have the PST
applied as it's already saved) rather than to the content retrieved
from the edit form (which should have PST applied).

Bug: 66084
Change-Id: Ibc189a4f3fc805ba0810ab3df259411d9f6f9fd7