LPS 45658 -- Fix padding for dialog iframe

Padding is added in 3 places:
Style attribute (of the dialog iframe HTML element): 
<html class="dialog-iframe-root-node" dir="ltr" lang="en-US" style="padding: 15px;">

aui_deprecated.css (lines 883-884) (This rule is overridden by the style attribute)
.dialog-iframe-root-node {
padding: 15px; }

aui.css (lines 4959-4962)
@media (max-width: 767px)
.aui body {
padding-left: 20px;
padding-right: 20px; }

The first two are constant for all widths, and only the last source is causing extra padding for widths under 767px. This is because the iframe is another html document, with its own head/body elements. We can’t delete the rule because it’s necessary for the page body padding. I think it is best to add another rule to _responsive-767px-max.scss, where the third rule from above is actually written.