Move mixins awaiting merge into helpers

- Move phase banner into layout file, since this is not provided as
part of the govuk template
- Move phase banner mixin (awaiting merge into front end toolkit) into
helpers file
- Both cookie message and footer alignment are awaiting the merge of
these two pull requests
1) https://github.com/alphagov/govuk_frontend_toolkit/pull/100
2) https://github.com/alphagov/govuk_template/pull/70