1741: Make rendering of iframed video embeds better in the Video page

[stevejalim]

This changeset improves the accessibility of embedded videos and fixes an overlooked HTML validation error.

(Related issue #1741)

Key changes:

• Adds a title attribute to the iframe, based on the page title, but only if there is not already a title present
• Drops the now-deprecated frameborder attribute, instead using CSS to ensure the iframe has no border

Example:

BEFORE <iframe width="480" height="270" src="https://www.youtube.com/embed/xTH0hb1nYoE?feature=oembed" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

AFTER: <iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" height="270" src="https://www.youtube.com/embed/xTH0hb1nYoE?feature=oembed" title="Interview with Stan Leong" width="480">

How to test

• Code is on staging, please view source of a video page, such as https://developer-portal.stage.mdn.mozit.cloud/videos/coding-dark-mode-your-website/, and also run it through a HTML5 validator, such as https://validator.w3.org/