Fixes #3258.

Non-exclusive offers that only allow combination with specific other offers

These release notes were created in 6a03a0bc1f and 05b14af8af, but
somehow didn't make it into the master branch.

This is why http://docs.oscarcommerce.com/en/latest/releases/index.html
currently doesn't mention these releases.

Currently, if a valid basket line is marked for deletion, any other
invalid basket lines on the formset prevent it from being deleted.

Regression introduced in cca43aa4d1

Bumps [select2](https://github.com/select2/select2) from 4.0.8 to 4.0.10.
- [Release notes](https://github.com/select2/select2/releases)
- [Changelog](https://github.com/select2/select2/blob/develop/CHANGELOG.md)
- [Commits](https://github.com/select2/select2/compare/4.0.8...4.0.10)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [inputmask](https://github.com/RobinHerbots/Inputmask) from 4.0.2 to 4.0.8.
- [Release notes](https://github.com/RobinHerbots/Inputmask/releases)
- [Changelog](https://github.com/RobinHerbots/Inputmask/blob/5.x/CHANGELOG.md)
- [Commits](https://github.com/RobinHerbots/Inputmask/compare/4.0.2...4.0.8)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

So that migrations are not required for changes to the
"OSCAR_IMAGE_FOLDER" setting, for projects that have not forked the
"catalogue" app.

Bumps [select2](https://github.com/select2/select2) from 4.0.5 to 4.0.8.
- [Release notes](https://github.com/select2/select2/releases)
- [Changelog](https://github.com/select2/select2/blob/develop/CHANGELOG.md)
- [Commits](https://github.com/select2/select2/compare/4.0.5...4.0.8)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

``django-phonenumber-field`` supports the same versions of Python and Django that django-oscar supports, so this should be a safe upgrade.
Fixed failing test due to invalid phone number in fixtures.
References #3144

Also do some minor code cleanup for changes made in #3017.

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.