Make avatar URL end with .png for using CDN

Fixes #917
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/urls.py

@@ -683,9 +683,15 @@ urlpatterns = [
     ),
 
     # Avatars
+    # Compatibility URL, remove for 2.6
     url(
         r'^avatar/(?P<user>[^/]+)/(?P<size>(32|128))/$',
         weblate.accounts.views.user_avatar,
+        name='user_avatar_compat',
+    ),
+    url(
+        r'^avatar/(?P<size>(32|128))/(?P<user>[^/]+)\.png$',
+        weblate.accounts.views.user_avatar,
         name='user_avatar',
     ),