Update step by step patterns to latest

[NickColley]

Update the step by step pattern by copying the original scss files and changing the gem-c- namespace to app-.

Split the single step-by-step-navigation file into three that mirror the three in govuk_publishing_components for easier maintence in the future.

For the JavaScript, copy the govuk_publishing_component JavaScript, replace gem-c- namespace with app-, rename module to AppStepNav from Gemstepnav

I've also made the analytics function do nothing as it's not relevant for a prototype.

Finally do a small tweak to the related items divider width.

Comparison between the Kit and live examples

Step by step pattern

• Step by step pattern in the Kit
• Step by step pattern on GOV.UK

Start page with step by step pattern

• Start page with step by step pattern in the Kit
• Start page with step by step pattern on GOV.UK

[hannalaakso]

LGTM 👍 Reviewed by comparing what's in https://github.com/alphagov/govuk_publishing_components/tree/master/app/assets and checked that everything still works.

This is a change to /app so users with existing prototypes won't get it unless they manually make the changes. It doesn't feel like a big/important enough change to prompt all users to update their existing versions of kit, given the overhead of updating all the relevant files? What do you think @NickColley?

In the long term these files probably shouldn't live in app (out of scope for this PR).

[NickColley]

@hannalaakso yeah I agree I think the changes to the pattern are small enough that it's not worth adding instructions to manually update but we can reconsider this based on user feedback.