[FIX] web: iOS content in modal disappear

As can be seen in https://github.com/scottjehl/Device-Bugs/issues/8 a
combination of "-webkit-overflow-scrolling:touch", "position:relative"
and "overflow:auto" can have unexpected behavior on iOS.

The issue solved here is that writing in a form view inside a modal in
iOS (at least version 10.3) would make the whole form body invisible
until edition is ended.

This commit fixes this by adding a CSS rule which avoid the iOS bug.

opw-774396
closes #20314

addons/web/static/src/less/modal.less

@@ -52,6 +52,8 @@
 
                 .modal-body {
                     overflow: auto;
+                    // fix iOS issue https://github.com/scottjehl/Device-Bugs/issues/8
+                    -webkit-transform: translate3d(0, 0, 0);
                 }
             }
         }