Use greentext format when formatting blockquotes

An implementation of greentext that cooperates with markdown.

The hook in `text_formatter.js` edits markdown before it is rendered. (Adds a `\>` after `>`)

On the mobile version the markdown is rendered on the backend and the hook in `html.rb` edits the HTML instead (adding `&gt;` after line breaks)

There's also some CSS that makes it show the same way as on 4chan.