Added an additional constant for the plugin base url for blueprint plugins

src/octoprint/templates/index.jinja2

@@ -44,6 +44,7 @@
         <script lang="javascript">
             var BASEURL = "{{ url_for('index') }}";
             var API_BASEURL = BASEURL + "api/";
+            var PLUGIN_BASEURL = BASEURL + "plugin/";
             var GCODE_WORKER = "{{ url_for('static', filename='gcodeviewer/js/Worker.js') }}";
 
             var CONFIG_GCODEFILESPERPAGE = 5000;