detox is unmaintained and tox has a parallel mode

Django 2.0 reached end of life in April 2019.

* Implement basic abstract thumbnails generator backend

* Create `oscar_thumbnail` template tag in `image_tags`

* Add `easy_thumbnails` to `INSTALLED_APPS` in sandbox settings

* Modify `primary_image` method of `Product` model

* Override in templates Sorl's `thumbnail` tag with `oscar_thumbnail` tag

* Check that related module is installed during thumbnail class initialization

* Add params to `original` field of `ProductImageFactory`

* Add media related URLs to test settings

* Add unit tests for thumbnailer backend

* Create test helpers in `test/utils`

These helpers will be used in tests related to thumbnails

* Add tests to check `oscar_thumbnail` template tag

* Add test to check images/thumbnails deleted for deleted product

* Move `sorl-thumbnail` and `easy-thumbnails` to `extras_require`

With this change these packages can be installed as
* `pip install django-oscar[sorl-thumbnail]`
* `pip install django-oscar[easy-thumbnails]`

* Updated "Template tags" section in docs with info about `oscar_thumbnail`

* Describe `OSCAR_THUMBNAILER` setting in "Settings" section of the docs

* Add information about created thumbnail backend to the release notes

* Add Django 2.2 to the Tox environments list.

* Add Django 2.2 to the Travis environment list.

* Mention in the release docs about Django 2.2 support.

* Upgrade to Django 2.2 in the setup requirements.

* Do not decode encoded base64 string, if it is not bytes instance, which changed in Django 2.2.

* Run Travis CI builds on Ubuntu Xenial, which has SQLite 3.8.3, required by Django 2.2.

* Add Python 3.7 to the test matrix.

* Update comment for the Django slugify function call, since it does not return `SafeString` since Django 2.2.

- Move Oscar app configs into Django app config classes
- Rename Oscar/Django apps' app config module
- Remove "oscar.get_core_apps()" and its app overriding
- Remove "oscar.OSCAR_CORE_APPS"
- Port app forking code to work with Django app registry
- Port class loading code to use app configs to get app properties

get_available_shipping_methods must always return an iterable.