Automatic insertion of report a problem

Once https://github.com/alphagov/static/pull/441 is merged and deployed, this change will allow an opt-out, zero-configuration way to add the `report-a-problem` form to any GOV.UK front-end apps. The feature can be opted out of by setting the `Slimmer::Headers::REPORT_A_PROBLEM_FORM` header to `false`.

This is a (slightly) breaking change. When upgrading `slimmer` for any app which currently has the report-a-problem form, the following need to be removed:
* `div#report-a-problem`
* any extra CSS for aligning the form or the form toggle

If this isn't done, there'll be an empty div in the page.
done (09e1453)