Let's remove it for now. Only contributors to the blog need that link, which can be placed i.e. in the footer, or in the page explaining how to contribute a blog post.
Instead, and also for now, we can add to the sidebar a link "Join Wikimedia Space" pointing to https://discuss-space.wmflabs.org/t/how-to-join-wikimedia-space/113
All these are interim solutions while waiting for T215052: Add MediaWiki login support to Discourse.