feat: [FC-0044] Course unit - Edit iframe modal window

[PKulkoRaccoonGang]

🚨 Dependencies:

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Description

This PR adds logic to support the functionality of editing xblocks via the legacy modal editing window.

Useful information to include:

• Communication between the Backbone.js and the React app is done via post messages.
• xBlock resources (CSS, JS files) are passed to the legacy modal edit window template.
• Added some styles for xblock legacy modal content.

Supporting information

The next PR will be adding post messages for the tagging functionality.

Testing instructions

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