Finalize post publishing/draft process

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For new posts

1. User starts typing
2. Periodically, the post is saved to the API as draft
3. API transforms markdown into html, sets it to post body, returns post with body filled in
4. Ember can show preview using body field in separate tab/pane
5. Once user clicks publish, the final version of the post is saved in the published state

For edited post

1. User clicks edit on existing post
2. Periodically, the post is saved to the API as a separate draft
3. API transforms markdown into html, sets it to post body, returns post with body filled in
4. Ember can show preview using body field in separate tab/pane
5. Once user clicks publish, the original version of the post has changes saved.

Questions

1. What does the API do with the drafts once publish is done - might require issue on API side.
2. How does the API figure out which original post the draft of an edit belongs to? It could be doable by identifying via number, but I'm not sure. There is no unique index on the number field at the moment, so it is possible.

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Blocked by #47

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Closed with #63