a11y: manage focus appropriately for modals with iframes in IE (LMS-1539...

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When an iframe loads in IE, focus automatically shifts to it. This poses a problem for two modals in the wiki that load with iframes, since focus does not shift to the modal's close button upon opening. This hack gets that to happen. It's based off of http://stackoverflow.com/questions/15792620/how-to-get-focus-back-for-parent-window-from-an-iframe-programmatically-in-javas.

@caesar2164 
@nedbat