Bring Guide up to date with changes from Envato Market Styleguide

[lukearndt]

This change brings across a bunch of improvements that we have put into the Envato Market Styleguide over the past few months.

Features include:

• [x] Support for viewing a scenario in whatever language you want, provided it's a supported language and there are relevant translations in place.
• [x] Plugging in whatever authorisation system you use is a lot cleaner.
• [x] The Guide::EndpointStocktaker, which can generate a list of all scenario endpoints (intended for automatic regression testing, experiments ongoing).
• [x] Support for Fixtures, which can be used to generate Guide::ViewModels in a reusable fashion.
• [x] Domain clarification; nodes formerly called Components are now called Structures.
• [x] A lot more things are now configuration options instead of hard-codes.
• [x] General tidy-ups and bug fixes.

[ppj]

:+1: based on a couple of caveats:

1. I have paired on a tiny part of it
2. As long as all/most of the changes are brought over from the Market styleguide :smile:

[zubin]

Apart from minor nitpicks, LGTM! :+1: (pending green of course)

[lukearndt]

Thanks for the thorough review @zubin !