[core] Display a warning when users is accessing a Hue 3 page while having Hue 4 on

This is similar to:
https://issues.cloudera.org/browse/HUE-6162

The condition to check if the user is supposed to be on Hue 4 is the same as
https://github.com/cloudera/hue/blob/master/desktop/core/src/desktop/views.py#L345

The goal is to notify users that they should update their bookmarks and urls to the new Hue (we met a series of users that were still on Hue 3 UI by mistake)

Let's also add a backend logging message so that we can track it.