Update checkout payment detail template: cleanup archived packages, replace with the relevant ones.

Restore voucher set URLs

As part of fbf8bac4c5/, the Voucher set URLs disappeared.

The following exception is logged on every Dashboard page:
`Reverse for 'voucher-set-list' not found. 'voucher-set-list' is not a valid view function or pattern name.`

Doc Fix: Link to stable documents in front README page

Update django-extensions to 2.1.9

Upgrade CDN version of jQuery

https://github.com/django-oscar/django-oscar/pull/3018 updated the in-tree versions

Add sorl-thumbnail to docs requirements

Update isort to 4.3.20

Update django-extensions to 2.1.7

Update werkzeug to 0.15.4

Update requests to 2.22.0

Add less.js to Dashboard Layout

* Retrieving product options now performs only 1 query.

Also it returns all the product options combined. This makes more sense because
as the existing tests prove, there is already a way to retrieve the product_class
options and the product options separately.

Update translatable strings

OSCAR_USE_LESS = True doesn't work in Oscar Dashboard. This commit makes it work by putting less.js on dashboard/layout.html

The version we are currently using is out of date, and the Transifex API
has changed to use API keys instead of username and password.

Prepare for 2.0b1 release

Improve release notes and change deprecation warnings

Update translations from transifex.

Drop Django 2.0 from test matrix and update supported versions in several places

Django 2.0 reached end of life in April 2019.

Remove `OSCAR_MAIN_TEMPLATE_DIR` setting.