Add a link to try download from another mirror

For example, we could replace the “I already downloaded Tails 3.8” link which becomes meaningless once people clicked on “Download Tails”.

Still, we should make it visibly distinct enough so that people notice that there’s a new link.

And make it possible to do metrics on it so we can prioritize #15755 better.

Parent Task: #15750

Original created by @sajolida on 15754 (Redmine)

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