Ambient-toolbox

* Moved changelog to "CHANGES.md" to make Dependabot understand/find it

* Fixed bug with filtering in EmailTestService for subject passing a gettext_lazy object (second occurrence)

* Fixed bug with filtering in EmailTestService for subject passing a gettext_lazy object

* Fixed broken init file

* Added `SaveWithoutSignalsMixin` model mixin

* Added `Selector` base class
* Added `AbstractUserSpecificSelectorMixin` and `GloballyVisibleSelector` helpers for selector pattern
* Added documentation for new selector pattern
* Added `django-upgrade` to linting
* Added date helper `get_first_and_last_of_month`
* Updated linting tools `black` & `pyupgrade`
* Added `pyupgrade` and `django-upgrade` to pipeline