Antoni

[Zendesk](https://govuk.zendesk.com/agent/tickets/5600748) [Live url](https://www.gov.uk/student-finance-calculator)

⚠️ This repo is Continuously Deployed: make sure you [follow the guidance](https://docs.publishing.service.gov.uk/manual/development-pipeline.html#merge-your-own-pull-request) ⚠️ Follow [these steps](https://guides.rubyonrails.org/upgrading_ruby_on_rails.html) if you are doing a Rails upgrade.

This PR replaces all links to the Whitehall ETA guidance (http://www.gov.uk/guidance/apply-for-an-electronic-travel-authorisation-eta) across the 'Check if you need a UK visa' smart answer, with links to the Mainstream ETA guide (www.gov.uk/eta)....

[Bug]: Images hosted on imgur now no longer work in the UK

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### Summary Imgur has withdrawn from the UK in response to the [Online Safety Act (OSA)](https://www.bbc.co.uk/news/articles/c4gzxv5gy3qo). This breaks all images in the documentation/tutorials that attempt to load from Imgur. For...