Research: Should we be using ProseMirror instead of MediumEditor?

Link: http://prosemirror.net/

ProseMirror looks more established and production-ready, with [great reviews](https://news.ycombinator.com/item?id=18998042). We should take a look at the pros and cons of each and decide on one to use long-term.

It's also used in several established platforms like the NYT and Zoho's new [writer5](https://www.zoho.com/writer/new-in-writer5.html).

Start by reviewing the ProseMirror link and the existing MediumEditor integration, though the issue names no repository files or tests. Compare both editors’ pros and cons, then document a decision about the long-term editor and the resulting implementation direction.

Written by the indexing model from the issue text.