Syntax highlighting in Markdown (or HTML) editor

Right now our `markdown` widget has a visual and raw mode, but the raw mode is just a wrapper around a textarea.

We should integrate [CodeMirror](https://codemirror.net/) as the raw editor, so we can do syntax highlighting of markdown and HTML.

See @ohadpr's PR #203 for more context.

Start with the markdown widget's raw mode and review @ohadpr's PR #203 for the existing context. The work is done when the raw editor uses CodeMirror and provides syntax highlighting for Markdown and HTML instead of only wrapping a textarea.

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