Make SmartAnswer visualisations feel like part of GOV.UK

>As a user arriving at https://www.gov.uk/check-uk-visa/visualise etc I need to know that it's from GOV.UK so I'm not confused or frigthened.
>
> -- *https://www.pivotaltracker.com/story/show/72160100*

This applies standard GOV.UK branding to @heathd's graph visualisation, while this tool is not publicly linked and will be used by GDS staff it's important it looks and feels like the rest of GOV.UK. 

Thats done by using standard header, footer, typography, buttons, etc. Friendly language, ensuring a user can get back the to actual Smart Answer and providing to Alpha banner to make clear this is not a prime time feature.

**Notes**:
* Updated `Slimmer` to get Alpha Bar functionality
* Apply module pattern to the JS to avoid globals
* Because the standard SmartAnswer/Format page CSS has some aggressive defaults this essentially resets, by using the 'simple' (`header_footer_only`) template and doing it's own gutters/styling.
* Other than modularising it and changing the appearance i've intentionally left the rest of the JS alone, it's out of scope of this story (and a potential rabbit hole).
* If you want to look at JS changes, see https://github.com/alphagov/smart-answers/commit/8ef6e6d0bf1e1e5184cf2dd41105a2f977899279 and https://github.com/alphagov/smart-answers/commit/2587deecc2194cca8a25c68965795283dd4c4a92 specifically, as the modularisation makes the file diff useless.