Bug 48273: Show last modified again

prepareCommonData runs after
SkinMobileBase::outputPage thus runs after
SkinMobile::prepareTemplate
thus runs after SkinMobile:prepareTemplatePageContent
hence sets postbodytext to an empty string

Lets conditionally set it if not set

Change-Id: Icf68513843fda6707850dc0a294b6fdab3c6b7d0
Merge "Bug 48273: Show last modified again"