Fix appurl error when 'page' does not exist in template contexts
Fix setup.py and bump dependency versions

* Enforce `django-polymorphic-tree` 0.8.4 to have a fixed list display.
* Enforce `django-fluent-contents` 0.8.4 to have layout fixes.
* Fix Django in install_requires
* Set version 0.8.1
* Sync with `setup.py` changes in other django-fluent packages.