Add a direct download link under the Install button in the Tails sidebar

See https://labs.riseup.net/code/issues/11269#note-11

Please add the link you added to the install page under this green button too. If you consider it necessary, state on top the download page something like “This is insecure, please consider using installation assistant”.

This should imo link to https://tails.boum.org/install/download/ and maybe even to https://tails.boum.org/install/download/openpgp/index.en.html

Related issues

Related to #11269 (closed)
Related to #11604 (closed)
Blocked by #11024 (closed)
Blocked by #11493 (closed)

Original created by @u on 11494 (Redmine)

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