Updated markdown renderer, added localhost:* to CSP connect-src

[tjcouch-sil]

• Added localhost:* to support local servers - thanks @hahn-kev for testing this out for us!
• Updated markdown renderer to latest in attempt to fix https://github.com/quantizor/markdown-to-jsx/issues/608
• Added anchorTarget prop to markdown renderer to enable people to choose to set to _blank. We can always change this decision later (make it default?), but I figured I'd just throw something in as a first step for now. See #1266

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