Code cleanup inspired by PR #34

Moving the compatibility switch to the fluent_pages.utils.compat code.
Also applied this in django-parler
Apply 2 fixes for moving pages.

- detection of parent change was broken by django-polymorphic-tree < 0.9
- changes to suburls are not saved to the proper language

Open issue: when subpages have a language which is not found in the
top-level.