Go to mapping works with sites

- Make the go to mapping dialog box (using `MappingsController#find_global`) find sites AND mappings instead of just mappings. Saves on a long list of sites in a table somewhere.

Pull requesting this as the tests pass fine locally, but consistently fail on five features when on Jenkins. Everything's rebased and up to date, and it's making no sense. It's probably something really tiny...
In the interest of getting these two improvements into the app, let's revert `js-go-to-site-or-mapping` to `js-go-to-mapping` and add a comment to explain why it is inconsistent.

As an aside, changing just the class name on current master still breaks the tests: https://github.com/alphagov/transition/compare/rename_class_experiment