Add CC sticky sidebar headers feature

This commit introduces a feature which makes the headers in the CC
sidebar remain visible (by sticking at the top) when scrolling down.

The idea was taken from the suggestion made at pekb/thread/60784834.

Change-Id: I6bbab04da855dbfa35ff8f8b1bbbc6d15946c36c
diff --git a/src/_locales/es/messages.json b/src/_locales/es/messages.json
index 2d075a1..38798b1 100644
--- a/src/_locales/es/messages.json
+++ b/src/_locales/es/messages.json
@@ -63,6 +63,10 @@
     "message": "Incrementa el contraste entre los hilos leídos y no leídos en la Consola de la Comunidad.",
     "description": "Feature checkbox in the options page"
   },
+  "options_stickysidebarheaders": {
+    "message": "Hacer que los encabezados de la barra lateral de la Consola de la Comunidad se queden pegados arriba (+info en <code>pekb/thread/60784834</code>).",
+    "description": "Feature checkbox in the options page"
+  },
   "options_save": {
     "message": "Guardar",
     "description": "Button in the options page to save the settings"