Avoid purging release from CDN multiple times in single request

In case several files are added, it's quite easy to hit CDN limits. Also
it makes no sense to fire several same purge requests at once.

Fixes phpmyadmin/phpmyadmin#13886

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

files/models.py

@@ -80,6 +80,8 @@ class Release(models.Model):
     snapshot = models.BooleanField(default=False, db_index=True)
     date = models.DateTimeField(db_index=True, default=timezone.now)
 
+    purged = False
+
     class Meta(object):
         ordering = ['-version_num']
 
@@ -432,6 +434,9 @@ def dockerhub_trigger(tag):
 
 @receiver(post_save, sender=Release)
 def purge_release(sender, instance, **kwargs):
+    if instance.purged:
+        return
+    instance.purged = True
     purge_cdn(
         # Pages with _littleboxes.html
         reverse('home'),