New admin mixin for adding translation helpers to the change form.
Simulate the admin popup to show the original english version of the content.
Add links next to translatable fields.
Plug the TranslatableMixin into the page/form admin classes.
Use the translation admin for links as well.
Add tests for swapping the rendering of the change form for the translated pages. Fix an edge case.
Don't build the links next to all the fields. Keep the definition of the translated_fields since it will be used/needed for #146
Merge pull request #148 from mozilla/145-original-content-links

Original Content Links