fix: open external links in new tabs

[Thanhdn1984]

Closes #176.

What changed

• Opens external links in new tabs for contract forms, blog items, Golem challenge links, and org job links.
• Adds/keeps rel="noopener" for external targets.

Notes

• I couldn't run mix format locally on this worker because mix is not installed here, so this is a small static patch.

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