Fix specs to work with new `static` version

Recent change in `static` ([1](https://github.com/alphagov/static/pull/441),
 [2](https://github.com/alphagov/static/pull/445)) changed how
the `report-a-problem` mark-up and JS code work, which meant that some specs in
this app broke. This change fixes those specs.
Bump slimmer to v5.0.0

This picks up the automatic report-a-problem form, which means that the form
can be removed from this app (and the appropriate header set in places where
it's not desired).