MDL-44361 Import YUI 3.15.0

Includes @VERSION@ fix
MDL-42960 Javascript: Expose upstream YUI logs to developers.

In order to expose YUI deprecation notices and other debugging when we are
developing, this patch changes our logging behaviour. With this change:
* all messages with a log level above 'warn' are displayed;
* specifying a yuiloginclude:
** changes the log level to 'debug';
** only displays the log sources included in the specified setting; and
** adds the empty ('') log source.
* specifying a yuilogexclude:
** changes the log level to 'debug'; and
** only displays the log sources included in the specified setting.

With effect from this change, if you are debugging a specific module you
will either need to:
* change the $CFG->yuiloglevel to 'debug'; or
* specify an appropriate value for $CFG->yuiloginclude; and/or
* specify an appropriate value for $CFG->yuilogexclude.