refactor: remove thumbnail for non-edX videos & allow removing fallback URLs

[bra-i-am]

Description

This PR aims to solve the issue openedx/frontend-app-authoring#1016 by showing the thumbnail from the videoSettingsModal only when the Video ID is one given by edX according to this method

Any other Video ID will hide the thumbnail:

Additionally, this PR fixes a bug when trying to delete a Fallback video URL:

[openedx-webhooks]

Thanks for the pull request, @bra-i-am!

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[bradenmacdonald]

@bra-i-am Thanks for contributing the fix! We've just moved this repo into https://github.com/openedx/frontend-app-course-authoring/ though; would you mind opening the same fix PR against that repo? Sorry for the extra work.

[openedx-webhooks]

@bra-i-am Even though your pull request wasn’t merged, please take a moment to answer a two question survey so we can improve your experience in the future.

[bra-i-am]

hey @bradenmacdonald, thanks for your answer. It's no problem. I already created the PR there: openedx/frontend-app-authoring#1241