Fix GFM video element hardcoded width

Merged Elias Steurer requested to merge (removed):master into master

What does this MR do?

Fixes #17139

When including a video in GFM the width is set to a fixed 400. This results in a broken mobile view and a strange looking desktop layout.


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With the width set to 100%:

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Does this MR meet the acceptance criteria?


This MR only alters only one value. If you need additional informations for this MR pls comment!