Merge models Page with Node

[jrief]

Up to version 3, there where 2 Page objects per Node object, hence a separation of these models made sense. In version 4 this is not the case anymore. For each Node there is one and only one Page object. For simplicity and performance it therefore would be a good idea to merge these two models into one; this eliminates one INNER JOIN on many queries.

[fsbraun]

I support this for django CMS 4.2. Tricky bit will be backward code compatibility.