Fixed wrong creation of select2tags components.

Now users can supply data to the attribute "data-select2-tags" to provide
existing tags.

flask_admin/static/admin/js/form.js

@@ -63,7 +63,7 @@
       this.applyGlobalStyles = function(parent) {
         $('[data-role=select2]', parent).select2({width: 'resolve'});
         $('[data-role=select2blank]', parent).select2({allowClear: true, width: 'resolve'});
-        $('[data-role=select2tags]', parent).select2({tags: [], tokenSeparators: [','], width: 'resolve'});
+        $('[data-role=select2tags]', parent).select2({multiple: true, tokenSeparators: [','], width: 'resolve'});
         $('[data-role=datepicker]', parent).datepicker();
         $('[data-role=datetimepicker]', parent).datepicker({displayTime: true});
       };