[CORE] Add custom favicon configuration support

Adds support for custom favicons defined in config.php that override theme favicons.

DOCUMENTATION/SYSTEM_ADMINISTRATORS/CONFIGURE.md

@@ -146,6 +146,7 @@ This section is a catch-all for site-wide variables.
     be fine-tuned for notices, messages, profile bios and group descriptions.
     Zero indicates no limit.
 
+* favicon: the path to a custom favicon, eg: favicon.png
 
 db
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lib/action.php

@@ -249,7 +249,7 @@ class Action extends HTMLOutputter // lawsuit
         if ($this->needLogin) {
             $this->checkLogin(); // if not logged in, this redirs/excepts
         }
-        
+
         if ($this->redirectAfterLogin) {
             common_set_returnto($this->selfUrl());
         }