Upgrade yuilib to the latest version

Copied from MDL-44929 with minor modifications. ----------------------------------------- Read https://github.com/yui/yui3/wiki/YUI-3.18.1-Change-History-Rollup in case you can think of anything I haven't considered. At present, the only changes which affect us at present relate to drag/drop. These changes must be tested on as many devices are possible - at least: android iOS Edge Chrome Firefox Safari So in summary: Ensure that a complete run of behat has been completed Ensure that drag/drop works (with no new bugs) (on all supported devices) for: block drag/drop activity/resource moving uploading of content to courses (Android, iOS) Moving dialogues around the screen (those which are draggable) The event listing tool (sorting, etc) The Activity Chooser appears in the centre of the screen The filepicker dialogue and sub-dialogues are stacked appropriately and appear in the screen nicely The fliepicker when called via an TinyMCE / Atto editor -> image