Pluggable XBlock React Editors

[kdmccormick]

Currently, Problem, Text, and Video components have built-in editors with a "nice" UX, and all other editors render a less integrated legacy editor UI in an iframe modal (besides ORA2 which has its own full MFE). We should instead have a plugin system that allows xblock editor code to live elsewhere but still be integrated with this repo.

This relates to:

• Tracking Issue: Refactors for Visual Editors #2088 (editors cannot be plugin based if they share a single global redux state)
• feat: add games xblock editor #2656
• The general problem of how to modernize XBlock UI rendering (learner view and author view).

[kdmccormick]

thanks for adding details Braden!