Render Markdown client-side with a <Markdown> component

[c-lliope]

See https://github.com/rexxars/react-markdown

[c-lliope]

Currently, we store markdown in the database and parse it into HTML in app/views/blocks/show.html.erb, using this helper.

We should render it on the client side for a few reasons:

• A smaller chance of a cross-site-scripting (XSS) attack
• In the future, this will let people preview their markdown before submitting it
• Fewer dependencies on the server
• Faster page load times

[c-lliope]

• [x] We should use the component's escapeHtml option, to prevent XSS attacks.