Add Select2 v3.2 to tree

Select2 is relatively more advanced compared to Chosen.

Select2 is distributed under Apache License, Version 2.0

flask_admin/static/select2/LICENSE

+Copyright 2012 Igor Vaynberg
+ 
+Version: @@ver@@ Timestamp: @@timestamp@@
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use this work except in
+compliance with the License. You may obtain a copy of the License in the LICENSE file, or at:
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed under the License is
+distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and limitations under the License.
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