Fix certain links not working properly in Markdown fields

Partially addresses #360.

Although this fixes the specific instance where mailto: links won't work, it doesn't solve the underlying problem that the way we handle these links is terrible. Ideally, we'd replace the links when publishing a markdown piece instead of using this sketchy handling every time we display markdown in the frontend. Or at least properly detecting if it's an embedded image or a hyperlink would be an improvement.