feat(etherpad): add optional admin web-UI access

Enables optional access to Etherpad's web-UI. This is useful for
managing Etherpad plugins.

Among other things, plugins makes it easy to manage/delete pads if you
install the adminpads2 plugin.

roles/matrix-etherpad/templates/settings.json.j2

@@ -71,6 +71,14 @@
     "chatAndUsers": false,
     "lang": "en-gb"
   },
+{% if matrix_etherpad_admin_username != "" and matrix_etherpad_admin_password != "" %}
+  "users": {
+    "{{ matrix_etherpad_admin_username }}": {
+      "password": "{{ matrix_etherpad_admin_password }}",
+      "is_admin": true
+    }
+  },
+{% endif %}
   "padShortcutEnabled" : {
     "altF9": true,
     "altC": true,