Add signup links index to data_attributes

[JamesCGDS]

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What

On finder pages, signup links can appear at the top and bottom of the page and they are rendered as separate modules. For GA4 analytics purposes, however, we want to treat these separate modules as if they were in the same module and combine their index values.

For example, if there are 2 signup links at the top of the page, their current index properties are index_link: 1/2, index_total: 2. Similarly, if there are a further 2 signup links at the bottom of the page, their current index properties are also index_link: 1/2, index_total: 2. We want to combine these index properties so that they become index_link: 1/2/3/4 and index_total: 4.

This PR calculates the index values in app/presenters/signup_links_presenter.rb and adds them to the data_attributes hash, which ultimately sets the data-ga4-index attribute in the govuk_publishing_components/components/subscription_links component.

Credit for the code used in this PR belongs to @andysellick ⭐

Examples:

• finder with two links (both Get emails and Subscribe to feed)
• finder with one link (Subscribe to feed)

Why

Trello card

Visual changes

N/A