Load plugin versions menu always from latest (#1543)

Always use the same version of the plugin versions menu script, loading
from the latest across all releases. This is to allow for updates to
this component to be deployed across any release at the same time to
allow for a consistent switching experience.

docs/_includes/header_custom.html

@@ -1,3 +1,7 @@
 <script src="{% asset "js/site_constants.js" @path %}"></script>
 <div id="plugin-version-menu" class="site-footer"></div>
-<script src="{% asset "js/plugin_versions_menu.js" @path %}"></script>
+{%- assign basePath = "" %}
+{%- if site.baseurl.size != 0 %}
+{%-   assign basePath = "/vagrant-libvirt" %}
+{%- endif %}
+<script src="{{ basePath }}/assets/js/plugin_versions_menu.js"></script>