Add workaround for width calculation of with empty Select2 fields

See https://github.com/ivaynberg/select2/issues/426

flask_admin/static/js/form.js

@@ -3,10 +3,10 @@
       this.applyStyle = function(el, name) {
         switch (name) {
             case 'select2':
-                $(el).select2();
+                $(el).select2({width: 'resolve'});
                 break;
             case 'select2blank':
-                $(el).select2({allowClear: true});
+                $(el).select2({allowClear: true, width: 'resolve'});
                 break;
             case 'datepicker':
                 $(el).datepicker();
@@ -49,8 +49,8 @@
       };
 
       this.applyGlobalStyles = function(parent) {
-        $('[data-role=select2]', parent).select2();
-        $('[data-role=select2blank]', parent).select2({allowClear: true});
+        $('[data-role=select2]', parent).select2({width: 'resolve'});
+        $('[data-role=select2blank]', parent).select2({allowClear: true, width: 'resolve'});
         $('[data-role=datepicker]', parent).datepicker();
         $('[data-role=datetimepicker]', parent).datepicker({displayTime: true});
       };