fix: separate initialValue to not cause re-render on updates

[eemaanamir]

Issue: wikimedia/edx-platform#599

Description

This PR fixes re-render issue with TinyMCE which cause lost in state and the cursor position

Adding any text in course overview section in Schedule and Details Page causes the cursor to go back to initial point or at the very start of the editor.
This is because we are changing the initialValue of the Editor when we add/update any text which causes re-render.

Reference: https://www.tiny.cloud/docs/tinymce/5/react/#initialvalue

Testing instructions

Steps to Reproduce

• Go to Studio Schedule and Details.
• Go to Course Overview
• Enter text in field

Expected Behavior

• Expect to be able to enter text in field

Actual Behavior

• When enter text, the cursor pops back to the 1st position in the field at every character.