Added save indicator in the options page in Gecko

Firefox doesn't allow extensions to close the options page via
window.close, so this commit adds a "Saved" message each time the
options are saved.

In Chromium, the behavior continues to be the same as before (the
options page is closed after saving the changes).

Change-Id: I4f313f382b527e205ba1dc9976af53cb13a0cba6
diff --git a/src/_locales/es/messages.json b/src/_locales/es/messages.json
index 89941c0..8627dbb 100644
--- a/src/_locales/es/messages.json
+++ b/src/_locales/es/messages.json
@@ -71,6 +71,10 @@
     "message": "Guardar",
     "description": "Button in the options page to save the settings"
   },
+  "options_saved": {
+    "message": "Guardado",
+    "description": "Message which appears in the options page when the settings are saved"
+  },
   "inject_links": {
     "message": "Enlaces",
     "description": "Heading which we use before the 'previous post' link in a user profile in TW"