Outline path to creating and storing Hallo-compatible HTML-like data

[thibaudcolas]

... so it's easier to integrate into existing sites.

Note that we likely won't get to 100% parity with Hallo – up to us to figure out how close this has to get.

Start defining how to do it, then think of whether to develop it ourselves or not. Two paths:

• Let the JS handle this, with one of the other exporters available on https://github.com/nikgraf/awesome-draft-js
• Let draftjs_exporter do it, by configuring it differently.