Editor: Markdown support for older posts

[xiaoniuniu89]

Context

story of master feature #197

It would be nice if we can add support for older markdown posts in our new editor. I want to spike creating support for this to see what is possible.

Expected Behavior

1. older posts are read only
2. read only but user has option to convert to new format ( codemods for this?)
3. Our new editor supports markdown and json and the user can switch between the two as they prefare

Current Behavior

Older posts are saved as markdown strings. New posts are saved as json data. Loading old posts in the new editor will crash the app.

[xiaoniuniu89]

@NiallJoeMaher can you assign to me please