Refactor the plugin javascript template

- Can appear in "base.html"
- Only renders anything if there are actually plugins available for the page

InvenTree/templates/base.html

@@ -209,6 +209,9 @@
 <script type='text/javascript'>
 
 $(document).ready(function () {
+
+    {% include "panel/plugin_javascript.html" %}
+
 {% block js_ready %}
 {% endblock %}