Sidebar plugin: add allow-popups-to-escape-sandbox to iframe sandbox

With the current sandboxing rules if you have a link that opens in a new window (which is allowed) using `target=”_blank”` the window opens but it is restricted to the same sandbox as the original iFrame. Adding `allow-popups-to-escape-sandbox` should allow that new window to break out of the sandbox.

This is useful because once you are out of the sidebar iframe allowing confirm and alert pop ups no longer has the risk that they will be confused with popups from the container page. Allowing these as well as downloading etc on the new page would be useful in a number of cases.

I understand that you should be able to use the SDK and the `openUrl` function that should function similarly, but depending on the application and use case updating the `target` on urls could be simpler solution.

I can add this to the ideas board as well, but this seems like more of a bug or oversight since the sandbox already includes `allow-popups`.