Add inner and outer block mixins to align content

- Add a new mixin, outer block - to set a max width of 1020px and a
fixed with for IE8 and below
- Add a new mixin, inner-block, to set left and right padding to match
the header and footer
- Change phase banner to use inner-block mixin

Once these mixins are merged into the front end toolkit, they can then
be used the to align the cookie banner and footer in the govuk_template.

A test page showing this in action exists here:
http://govuk-elements.herokuapp.com/examples/template/