Standardise/streamline interactive embeds in DCAR

[frederickobrien]

One for us to consider for the coming quarter, it came up while chatting with @JamieB-gu that we might not need to support both interactive elements and interactive atoms. It's possible interactive atoms supersede interactive elements, and we should get rid of the latter.

There are quite a few components floating around, including:

• InteractiveAtom
• InteractiveLayoutAtom
• InteractiveBlockComponent
• InteractiveAtomMessenger

With DCAR for interactives on apps out the door it'd be nice to take stock of the full interactives setup in dotcom and streamline where possible.

Some research will be required to make informed decisions on this but a few places to start:

• Fix interactive embeds #4077
• Deprecate interactive block elements #13328