[DX] Change modals so special server whitelist is not required

This week while setting up a nice shiny new, secure server. I ran into a problem where the modals on my Backdrop site wouldn't open. In particular - the one for comparing config differences: http://www.example.com/admin/config/development/configuration/sync/diff/field.instance.node.article.field_whatever

We should fix this callback so that people building backdrop sites (and servers) won't need to whitelist 'unsafe-eval' to get this modal working.