models: change path construction in petal_directory (fixes #21111)

Use an md5 hash of the CUT.uuid instead of the first 4 characters which
are constant now.

petale/models.py

@@ -14,6 +14,8 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+import hashlib
+
 from django.db import models
 from django.contrib.auth.models import User
 from django.utils.translation import ugettext_lazy as _
@@ -111,7 +113,7 @@ def petal_directory(instance, filename):
 
     return 'data/{0}/{1}/{2}/{3}'.format(
         instance.partner.name,
-        instance.cut.uuid[:3],
+        hashlib.md5(instance.cut.uuid).hexdigest()[:3],
         instance.cut.uuid,
         instance.name)