Make document footer component handle rtl content

For RTL content change the text direction and swap the floats so the
reading edge is the other side of the page. There will be a future bit
of work for adding translations to the strings in the component.

This is dependant on [this enhancement][1] making it into the 
govuk_frontend_toolkit. It will need to be amend to use that once it is
available.

[1]: https://github.com/alphagov/govuk_frontend_toolkit/pull/151
I am fine with it, I don't think it will have much effect in this situation. The other option is to make it use `.govuk-document-footer-direction-rtl` class which seems a little... verbose.