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.