Adding iframe attributes to attribute whitelist

To properly show YouTube-embeds, we need to allow some attributes on `iframe` tags. I've added all attributes that are normal for such an embed, including `width` and `height` (which we may not need if we want to make the frame resize to its parent).

pydis_site/settings.py

@@ -373,6 +373,7 @@
     'img': ['class', 'id', 'src', 'alt', 'width', 'height'],
     'section': ['class', 'id'],
     'article': ['class', 'id'],
+    'iframe': ['width', 'height', 'src', 'frameborder', 'allow', 'allowfullscreen'],
 }
 
 WIKI_MARKDOWN_HTML_WHITELIST = [