Make any iframe embed responsive in the editor

[notnownikki]

Description

We used to reply on oEmbed's type information to make
YouTube embeds responsive in the editor. This change
removes that dependency, and looks for content embedded
in an iframe with a fixed size, and converts it to a responsive
iframe that keeps the correct aspect ratio when resized.

How Has This Been Tested?

1. Create a new post with embeds for all the test content listed in components/sandbox/README.md
2. Resize the browser window to a narrow width
3. The YouTube videos should resize and keep the correct aspect ratio.

Types of changes

Bug fix

[niranjan-uma-shankar]

This is of interest to the PR #4710 too, which uses the Sandbox component. Also, responsiveness is expected to work for any oEmbed video provider, right, for eg. TED, funnyordie etc.?

[notnownikki]

This is for providers who embed their content in an iframe with fixed width and height, which is what YouTube does. This same approach can be used with other elements too, so we can extend this for different fixed width elements of we need to

[notnownikki]

@niranjan-uma-shankar I just tested this with funnyordie, and they embed their content in a fixed size iframe, so this automatically applies there too. The iframe is initially set to the width of the editor, and as it gets resized it keeps to the width of the editor, and keeps the correct aspect ratio.

[notnownikki]

Part of #5530

[jasmussen]

This seems to me to be an improvement. Everything could always do with a better code review, than I can give. But given the state of embeds, I think we should consider merging this in so we don't get stuck. 👍 👍