CSS: Use auto for iframe body height

This change makes no visual difference right now, but will matter (for
reasons I don't understand) once we change `ace.js` to build the
iframes by constructing elements in JavaScript (vs. writing HTML).

src/static/css/iframe_editor.css

@@ -5,7 +5,6 @@
 @import url('./lists_and_indents.css');
 
 html.outer-editor, html.inner-editor {
-  height: auto !important;
   background-color: transparent !important;
 }
 #outerdocbody {

src/static/css/pad/layout.css

@@ -1,11 +1,12 @@
 html, body {
   width: 100%;
-  height: 100%;
+  height: auto;
   margin: 0;
   padding: 0;
 }
 html.pad, html.pad body {
   overflow: hidden;
+  height: 100%;
 }
 body {
   display: flex;