## Description
Fixes#591.
## Rationale
We are currently not testing many combinations, we run Flake8 on the generated project with default options, but that rarely catch any issues.
## Use case(s) / visualization(s)
Catch problems with invalid combinations, for instance, it fails due to Whitenoise breaking flake8 with `django-compressor` because `STATIC_URL` was undefined here:
b91c70d755/%7B%7Bcookiecutter.project_slug%7D%7D/config/settings/production.py (L185)
* Create a test matrix on Travis CI to help testing multiple options
* Change test_docker.sh to fail if any command in it fails
* Run black on the CI with --check option
* Fix formatting of project files using black
* Install black in the docker container
* Exclude migrations in black checks
* Fix Black formatting violations
* Run black on the whole generated project & fix issues
sorl-thumbnail serialized and deserializes the storage class and during this
process location parameter is lost if factory is used. Using a full-fledge
storage class fixes this issue.
See: https://stackoverflow.com/a/18046120/104731
* Convert old-styled string formatting to f-string
* Update flake8 explicit version to 3.5.0
* Make tox.ini in sync with requirements.txt
* Fix annoying flake8 F405
* Prettify and re-order settings entries
* Use old-style .format() for the time being
* Remove redundant linebreaks at the settings files' beginning
* Fix E303 too many blank lines
* Remove a redundant linebreak from requirements.txt
* Some linebreake juggling in config.settings.base
When creating a Mailgun add-on on Heroku, the app gets some environment
variables by default:
MAILGUN_API_KEY
MAILGUN_DOMAIN
However, the cookiecutter names do not match and requires a manual step
from the user deploying. It's used elsewhere but shouldn't harm the
other deployment methods to rename these variables.
While updating the docs I noticed a variable that appear unused
DJANGO_MAILGUN_SERVER_NAME so this removes it from the documentation.
Since config/production.py sets a new value for `DATABASES['default']`, the `DATABASES['default']['ATOMIC_REQUESTS'] = True` setting from base gets overridden. So it's probably a good idea to add it back.
