Hide canonical url for root page

The canonical url field in page properties should not be visible for the home page: users shouldn't be able to edit it since it should always be "/".

This is the result of this pull request (for the homepage, other pages aren't affected):

> ![no-canonical-url-for-home](https://f.cloud.github.com/assets/928116/317249/15bc6fbc-9862-11e2-9226-f11366e26551.png)