discourse.plugins: Make the updater able to package plugins

Let the update.py script handle the initial, repetitive task of
packaging new plugins. With this in place, the plugin only needs to be
added to the list in `update-plugins` and most of the work will be
done automatically when the script is run. Metadata still needs to be
filled in manually and some packages may of course require additional
work/patching.

(cherry picked from commit f8096460bd15d4f13a01cfddf0a30798921fdb42)

nixos/modules/services/web-apps/discourse.xml

@@ -284,11 +284,22 @@ services.discourse = {
       Ruby dependencies are listed in its
       <filename>plugin.rb</filename> file as function calls to
       <literal>gem</literal>. To construct the corresponding
-      <filename>Gemfile</filename>, run <command>bundle
+      <filename>Gemfile</filename> manually, run <command>bundle
       init</command>, then add the <literal>gem</literal> lines to it
       verbatim.
     </para>
 
+    <para>
+      Much of the packaging can be done automatically by the
+      <filename>nixpkgs/pkgs/servers/web-apps/discourse/update.py</filename>
+      script - just add the plugin to the <literal>plugins</literal>
+      list in the <function>update_plugins</function> function and run
+      the script:
+      <programlisting language="bash">
+./update.py update-plugins
+</programlisting>.
+    </para>
+
     <para>
       Some plugins provide <link
       linkend="module-services-discourse-site-settings">site