content/models.py: ensure uniqueness of page titles.

debian/changelog

-flyers (0.20140721) UNRELEASED; urgency=low
+flyers (0.20140722) UNRELEASED; urgency=low
 
   * Initial release.
 
- -- Christophe Siraut <d@tobald.eu.org>  Sun, 09 Nov 2014 13:40:06 +0100
+ -- Christophe Siraut <d@tobald.eu.org>  Tue, 18 Nov 2014 16:13:43 +0100