Updated required Django version
Updated Django version in depcheck
Cleanups
Bugfix: Undo bugs introduced in a previous commit
Docs: Styleguide additions

* More detailed imports order,
* More detailed order for models internals (fields, methods...),
* Guidelines on writing properties,
* Rewording of some other phrases, without altering their meaning.
Cleanups on models
Cleanups on properties
Cleanups on cached properties