Make default header image URL configurable.

r2/example.ini

@@ -52,6 +52,8 @@ system_user = reddit
 admins = reddit
 # the default subreddit for submissions
 default_sr = reddit.com
+# default header image url
+default_header_url = /static/reddit.com.header.png
 
 # time for the page cache (for unlogged in users)
 page_cache_time = 90

r2/r2/models/subreddit.py

@@ -737,7 +737,7 @@ class _DefaultSR(FakeSubreddit):
     #notice the space before reddit.com
     name = ' reddit.com'
     path = '/'
-    header = 'http://static.reddit.com/reddit.com.header.png'
+    header = g.default_header_url
 
     def get_links_sr_ids(self, sr_ids, sort, time):
         from r2.lib.db import queries