This:
1. Introduces a variable for the Course Outline view in Studio.
A custom theme can override it to add new editors.
2. Exports a function for creating new editor modals.
A custom theme can use it to create editors without adding boilerplate code.
3. Adds a pluggable override for XBlock fields that are passed to the Studio.
Without this, custom editors in Studio cannot retrieve values of XBlock fields.
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