UI: Modal dialogs do not work in IE8

0. Ensure conditional availability is turned on at server level. Using IE8: 1. Add a new Page. 2. Click the 'Insert image' button in editor. EXPECTED: Image dialog appears and rest of page is greyed out (after slight flicker). BEFORE FIX: Image dialog is greyed out and you can't click things in it. 3. Click the 'browser' button to open the file picker. EXPECTED: File manager appears on top of image dialog, which is greyed out now. (NOTE: Due to a separate IE8 bug, the left-hand part of the file picker appears transparent, but is still usable.) 4. Twiddle around with pages on the file picker as necessary, then upload an image and OK it. EXPECTED: The image dialog is now available again. 5. OK the image dialog EXPECTED: It closes and the page is now available again. NOTE: Using Atto, the image will not actually be added to the editor. This is an unrelated issue. 6. Expand 'Restrict access' 7. Click 'Add a restriction'. EXPECTED: Dialog appears and rest of page is greyed out, as above. 8. Click 'Date' button to add a date restriction. EXPECTED: Dialog goes away successfully. 9. Using a modern browser, repeat these steps. EXPECTED: Same results except no flicker. (I.e. behaviour is unchanged from before.)