Add check-external-links command

[owenatgov]

Change

Adds a check-external-links npm script as an offshoot of check-links, creates a manual workflow for it and attempts to fix some initially reported broken links.

This is a reimplementation of #3174 as a separate command. It's inadvisable to include this as a default part of our testing suite because:

• it takes a long long time to run, roughly 7 minutes
• it reports on things that aren't actually failues such as links that aren't redirecting or broken but maybe just return unclean responses or some assets that are intentionally not proper links
• it could raise test failures for PRs that have nothing to do with that PR's change

However a separate comamnd we can run mean we have this tool that we can interpret whenever we want.

An early draft of this PR included the script in our default test suite in order to capture our current broken links. See the original test output of check-external-links here.

The check additionally flagged broken links in our component docs which I've actioned in alphagov/govuk-frontend#5428

Controversial content updates

A few of the content changes I've chosen to make probably need further discussion. I'll note the tricky ones or lingering problems here:

• In the accessibility statement, we originally linked to the tec docs gem and its accessibility statement, however the website for the library has been taken down. I've instead linked to the github repo and the file in the repo for the statement since its not listed anywhere online.
• In the code of conduct pages, we link to what was a community policy for Practical Service Design's slack space. However, it looks like Practical are shutting down and have started removing pages, including this one. I've tried to remedy this by removing the link to them entirely.
• In the footer component, we originally link to a service manual page on publishing info about your services accessibility to help users with adding an accessibility statement. However that page now redirects to info on wcag 2.2. I've tried to remedy this by instead linking to the sample accessibility statement.
• On the create accounts and national insurance numbers patterns, we had a duplicate section on not using NI numbers as credentials, linking to a section in a service manual page as evidence. That section has now gone from that page. I couldn't find alternative content to fill the gap so I've removed both those sections.
• On the password input component in the section on not using the spellcheck attribute we mentioned 'spell-jacking'. The page we link to for this is a 404. In fact, it looks like Otto js has completely removed their news section. I tried looking for an alternative but wasn't confident in my choices. I've aleviated this by removing the link entirely.

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[romaricpascal]

The command works fine locally, but I think we need a little extra work on the GitHub workflow to avoid issues if the cache has been deleted. Happy to pair if it's useful 😊

[romaricpascal]

issue This makes the workflow dependent on the build existing in GitHub actions' cache. Caches expire after 7 days without being used.

I think we need to look at sharing the build job between the test and check-external-links workflow. This is something we do in govuk-frontend if we need 'inspiration' 😉

[owenatgov]

@romaricpascal I've had a go at this in 88178a8. I'm not feeling ultra confident about it, especially since js tests are failing and specifically calling out bad cahcing, which I suspect is the problem. Any advice?