Post Editor: add new Copy Post functionality

[Copons]

The Copy Post functionality will allow users to choose an existing post and use it as a template for a new one.
Title, content, taxonomies, custom fields and everything else will be copied into the draft, ready to be modified as needed.

My initial approach is to add a new accordion in the editor drawer, containing a searcheable list of all posts. Through it, users can find an existing post, select it and use it as a template for the current draft.

Test live: https://calypso.live/?branch=add/copy-post

[aduth]

• Should be consistent with your module import/export syntax. Specifically, I'd recommend using export default instead of module.exports
• I've been discouraging folks from using React.createClass, as looking through the React core meeting notes it becomes very obvious that it's a discouraged feature. Consider instead the React.Component class or, if your component doesn't need lifecycle or state, a pure stateless function component

[Copons]

Regarding both your annotations: I actually had to refrain myself from using export and class and stateless components, as the few Calypso files I opened to understand Calypso's conventions didn't use them!
I'll update the files as soon as possible!