Move report-a-problem related styles to _report-a-problem.scss

This allows apps which rely solely on `header-footer-only.css` to also
get the 'report-a-problem' toggle styles.
Report-a-problem improvements

These changes in markup and styling make the report-a-problem functionality much more
self-contained, thus removing the need for apps to include custom CSS
for displaying the report-a-problem form correctly.

The report-a-problem stylesheets are also refactored to use the latest and greatest from
`govuk_frontend_toolkit`.
Merge pull request #441 from alphagov/report-a-problem-improvements

Report-a-problem improvements