Online defaults to order:hot, with an Account toggle

New online_hot_default preference (on by default, so guests and accounts
that never saved preferences get it): opening /online without a search
replaces the URL with ?tags=order:hot — e621's metatag for the order the
Hot page uses — so it is visible in the search field and shareable.
Existing searches are never touched: they live in the URL, so a refresh
or back/forward keeps them, while a fresh visit (nav pill, first time,
after a long time away) gets the hot default again. Turning the toggle
off opens Online on the site-wide newest posts as before.

The toggle sits in Account -> Browsing preferences and saves with the
rest of the settings; the backend validates the new boolean
(400 for non-boolean input).

backend/apps/accounts/serializers.py

@@ -127,3 +127,5 @@ class PreferencesSerializer(serializers.Serializer):
     ordering = serializers.CharField(max_length=30, required=False)
     per_page = serializers.IntegerField(min_value=12, max_value=200, required=False)
     zoom = serializers.IntegerField(min_value=120, max_value=400, required=False)
+    # Open Online with the order:hot metatag when no search is given.
+    online_hot_default = serializers.BooleanField(required=False)