add tether, required for bootstrap

sourcing/templates/base.html

@@ -27,6 +27,7 @@
 </div>
 
 <script src="{{ url_for('static', filename='jquery/jquery.js') }}"></script>
+<script src="https://npmcdn.com/tether@1.2.4/dist/js/tether.min.js"></script>
 <script src="{{ url_for('static', filename='bootstrap4/js/bootstrap.js') }}"></script>
 
 {% block scripts %}