NHS data - add URL check

[kathsherratt]

NHS data sometimes doesn't update e.g. on bank holidays.

This PR adds a small piece in download_nhs_regions() to make the download more robust to recent missing data / broken links.

• Sets a date sequence of the 7 days prior to the user-input release_date (our existing default is Sys.Date()-1)
• Each date is fed through existing code to create the recent NHS url
• All URLs are checked and only working URLs kept (status code 200)
• The most recent working URL is set as nhs_recent_url, which continues in existing pipeline

Potentially fixes #370

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[seabbs]

Jumped in and squished those minor issues as so minor. Merging now - great stuff :)

[kathsherratt]

Thanks so much!