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`.

This work was done together with @edds.
oops - fixed. I saw the change while rebasing but somehow managed to not carry it across...