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