Locale change link and alternate meta tags for engines / pensions calc

Have attempted to build a clean way to encapsulate the details we need in order to mount an engine.

If we were to add another engine there would be two steps.

* Add a new class in the ``ToolMountPoint`` namespace, inheriting from ``ToolMountPoint::Base``, with ``EN_ID`` and ``CY_ID`` constants.
* Update ``EmbeddedToolsController#engine_name`` to recognise the namespace of the new controller.

Testing the controller / path generation stuff is a bit tricky so have just gone with adding to the feature for that.