fix: useEffect to set editor.tsx#isEditable

[ICEDLEE337]

Current behavior: Externally changing the value of isEditable doesn't trigger a re-render and editor remains in same state.

**New behavior ** Externally changing the value of isEditable triggers a re-render and editor responds appropriately.

Context For example, it may be desireable for consuming code to lock the editor until a "Checkout for editing" button is clicked upon which server-side state reflects the fact that a specific user is currently editing the content as a primitive way of preventing merge conflicts.