When user types tab, translate it into spaces for consistent indentation.
When user types tab, translate it into spaces for consistent indentation.

Before this, CodeMirror was storing literal tabs. They align in the IDE because it has tab size set to 2, but are inconsistent when saved to disk or viewed in another editor. The issue is discussed and my solution comes from this CodeMirror thread: https://github.com/codemirror/CodeMirror/issues/988

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