Improve mirror SSH host key detect/manual buttons UX

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In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10355#note_157915592 @cperessini mentioned that the "Detect host keys" and "Input host keys manually" buttons could be improved.

Are the 'detect' and 'input manually' buttons mutually exclusive? Does it make sense then to have the two buttons side by side ('Detect host keys or Input host keys manually) and replace the content below depending on which is clicked?