Don't let people clobber existing filenames (#80)

www/%participant_id/participant_id.json

@@ -1,3 +1,5 @@
+import os
+
 from aspen import Response
 from gittip import db
 from psycopg2 import IntegrityError
@@ -7,6 +9,7 @@ ALLOWED_ASCII = set("0123456789"
                     "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
                     ".,-_;:@ ")
 
+FILENAMES = os.listdir(website.www_root)
 
 # ========================================================================== ^L
 
@@ -30,6 +33,9 @@ for i, c in enumerate(new_participant_id):
         raise Response(400)  # XXX Burned by an Aspen bug. :`-(
                              # https://github.com/whit537/aspen/issues/102
 
+if new_participant_id in FILENAMES:
+    raise Response(400)
+
 
 # Persist
 # =======