Remove wp.element; embrace React

[nylen]

We already have a lot of React-specific code in various places in the editor, so the wp.element.createElement abstraction has been more confusing than helpful for a while now. It seems to me like we are trying to pretend that we're not tied to using React, but this is a giant, untested assumption. I see two possible ways forward here:

1. Test this assumption and solidify our abstraction by proving that it is possible to switch to a different view rendering engine.
2. Drop the assumption and call our use of React by the name React.

Per recent discussion in core Slack, WP core has basically decided to adopt React. We are now well-positioned to prefer option (2) and remove as much of wp.element as possible.

[aduth]

With React 16 noisily warning about using PropTypes and createClass (two top-level API members we'd targeted as discouraging with the abstraction), this might be reasonable to consider.

[ellatrix]

Agreed.

[dmsnell]

Embrace React, embrace ReasonML.