FieldList->insertBefore/After now accept arguments in either order (fixes #2737)

I ran through the rest of the framework and updated uses of `insertBefore()`/`insertAfter()` to the new order. The CMS only uses them on one occasion (`SiteTreeFileExtension->updateCMSFields()`).

I’m happy to break this into two commits if that’s preferred (one for the new ordering, one for updating uses in framework and docs).