FIX Respect escaping rules on readonly fields in CMS history view

Avoids showing unescaped HTML on fields which didn't allow it otherwise,
e.g. TextField. This also fixes problems with SiteTree->ExtraMeta
which got evaluated within the CMS. If this section contains any
JavaScript it gets executed and can disrupt CMS operation.
Revert "FIX: updateCMSFields not accepting var by reference"

This reverts commit e56a78b1605ea958d844b4efca507074d5050c71.