Update workflow for library-sourced content (Authoring MFE - Unit Page)

[bradenmacdonald]

1. When a component in a course is sourced from a v2 library and a newer version is available in the library, display the "Update Available" button. This is already implemented on the legacy Unit page; this task is porting it to the new MFE unit page.

2. When the button is clicked, display a modal with the <CompareChangesWidget>:

3. The modal should be large to facilitate previewing the components.
4. The previews should be interactive (this is already implemented)
5. The "Accept Changes" and "Ignore Changes" buttons should both work, as they already work in the legacy UI.
6. Use a Paragon modal for this. It will look different from the legacy UI screenshot seen above.

[bradenmacdonald]

@jmakowski1123 FYI

[jmakowski1123]

cc @GlugovGrGlib for visibility. These are small additions being made to the new unit page MFE.

[navinkarkera]

@bradenmacdonald Now that the course unit is an iframe that displays the legacy UI, is this issue relevant?

[bradenmacdonald]

@navinkarkera Yes, we should be intercepting the events from the iframe and displaying this modal from the Authoring MFE, not from the legacy page. See how they're doing it for the "Move XBlock" modal: #1422

[navinkarkera]

@jmakowski1123 @lizc577 @sdaitzman @marcotuts This is ready for AC testing on the sandbox