Add support for creating dialogs, and add an About dialog.

This adds bootstrap-dialog to help with creation of nice dialogs,
and adds the ability for modules to render Javascript into the
browser, and specify onclick handlers in the menu system.

Also add a basic About dialog, using the new infrastructure and
showing some useful info about the application.

web/pgadmin/templates/security/watermark.html

@@ -1,5 +1,5 @@
 {% block watermark %}
 <div style="position: fixed; bottom: 0; right: 0;">
-    <img src="img/logo-right-256.png" alt="pgAdmin 4">
+    <img src="{{ url_for('static', filename='img/logo-right-256.png') }}" alt="pgAdmin 4">
 </div>
 {% endblock %}