Automatically insert report-a-problem into the wrapper

Before this change, each app had to add a `div` somewhere
that `slimmer` replaced with the report-a-problem form. With this change,
the form is inserted as a child of the wrapper without any extra markup being
needed in the app itself.
Add header which disables report-a-problem form addition
Slimmer::Skin: refactor wrapper variable to avoid duplication
Merge pull request #85 from alphagov/automatic-insertion-of-report-a-problem

Automatic insertion of report a problem