We were already compiling the markdown bundle via ember-cli, but that version was only being used in the test environment. This commit improves the implementation, and updates the filename so it's also used in production.
This commit also
- Removes the vendored copy of `markdown-it.js` and fetches from node_modules instead
- Updates `pretty_text.rb` to remove the custom sprockets-manifest-parsing
- Removes `pretty-text-bundle.js`, which was only being used by `pretty_text.rb`
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