Use standard previewable Markdown input fields on admin settings page

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We've got three fields on the Admin settings page, about to be four, that say "Markdown enabled", but are otherwise very bare-bones:

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I think it'd be good if we made them like our other Markdown fields (such as New Issue) for consistency, and to get the Preview functionality.

"Attach a file" probably wouldn't be applicable, so we'll need a way to disable that functionality.

cc @jschatz1