add campaigns controller

allows for bespoke pages to live within the responsive site

cc @andrewgarner is this what you had in mind?
How about checking for the presence of a template instead of rescuing from the exception?

```rb
if template_exists?(params[:id])
  render template: params[:id]
else
  not_found
end
```

Also - we should reuse (or refactor) the `not_found` method, not introduce a similar way to achieve the same behaviour.