(bug): Video thumbnail change only updating thumbnail in users gallery

Summary

It appears that when setting a thumbnail for a video, it only applies on the media page; in other peoples news-feeds, it is still how it used to be.

Ideally we should also make this feature more readily available (e.g. on the edit on the newsfeed).

Steps to reproduce

1. Go to the full screen media page for an activity.
2. Click on the options menu (⋮).
3. Hit edit.
4. Set a video thumbnail and save.
5. Refresh and compare the media page thumbnail with the newsfeed in another window/tab.

Platform information

Universal Web.

What is the current bug behavior?

Thumbnail does not update for media feed.

What is the expected correct behavior?

Thumbnail changes should propagate across the site.

Relevant logs and/or screenshots

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Possible fixes

(If you can, link to the line of code that might be responsible for the problem)