Clean theme Iframes overflowing article container

[StephDriver]

This has been split off from #4261 as it has similar symptoms but not from the same cause.

Not just References - other content overflows too. This was noted while testing another fix on my dev install, so this article does not exist on Clean, it was the one flagged up for a different problem in #3911 then downloaded for use as a test subject, but it illustrates the problem.

Originally posted by @StephDriver in #4261

• content inside the article should be constrained by the article container, and not overflow into the sidebar.

[StephDriver]

This was discussed at backlog refinement today, and was split from #4261.

In a nutshell:
An iframe within the article should be constrained by the article width and not overflow into the sidebar.
This is a theoretical issue, as it was observed during testing, so the article in question is not published in Clean. But such a scenario could exist live.