Changed size of embedded videos in documentation

We've been embedding videos in topics, for example: https://docs.gitlab.com/ee/user/project/merge_requests/code_quality.html

The video is a bit big for the page, and it would be great if it could be a bit smaller (and allow the user to access full screen if they want).

Here are our instructions for embedding videos: https://docs.gitlab.com/ee/development/documentation/styleguide.html#embed-videos

I proposed some changes to the CSS based on the values YouTube gives you when you try to embed a video. I am no CSS expert--I just wanted to propose something to kick this off. (I know we don't like hard-coding values. I played with percentages but had no luck.)

The values in this MR may not be mobile-friendly, and we may want to center the video. I'm hoping someone with more experience can chime in and help.

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