This is recommended by the Django documentation

A new check in flake8 complains about them.

Previously inactive partners were shown in the API. As the Serializer does not include the is_active
field there was no tell whether a partner was active or not. Now the API only returns active partners.

Move notification email of vacancies to a management command instead of using a non-secured HTTP endpoint

If an expiration date is expired, it isn't shown on the site anymore.
If an expiration date expires within a month, a mail is sent to sponsor [at] thalia [dot] nu.

We chose not to add contact details, as they are part of Partner, not Vacancy.

Close #170

Also now does it with a list comprehension as the tuple just
consumes the generator anyway.

Thanks, you've served us well.

Closes #266

Fix #171