Add default icon to feeds

Every feed has an icon. If not set by the user, the database
automatically sets the default icon to media/default-icon-svg.

backend/app_be/models.py

@@ -9,7 +9,7 @@ class User(models.Model):
 class Feed(models.Model):
     url = models.TextField(blank=False, null=False, validators=[URLValidator(['http', 'https'])])
     active = models.BooleanField()
-    icon = models.FileField(upload_to='feed-icons', blank=True, null=True,
+    icon = models.FileField(upload_to='feed-icons', blank=True, null=False, default='default-icon.svg',
                             validators=[FileExtensionValidator(['png', 'svg'])])
     keywords = models.TextField(blank=False, null=False)
     match_all_keywords = models.BooleanField(blank=True, default=False)