Review Stage: make "Review Rating Field Name" and "Review Confidence Field Name" default to "rating" and "confidence"

[melisabok]

And add a preprocess function that checks if the field name exists in the default form + additional review form options - remove review form options.

The field must stay optional in case there are review forms with no rating/confidence.

Context: Some PCs change the review form but they forget to the define the correct rating and confidence fields.