This PR introduces a new DateField to required assets for due date. If it's past the asset's due date, the input will be disabled for the user to enter with data in the sponsorship detail page.

Besides that, the PR adds a new Django command to check for assets that will reach the due date in 7 days (by default, but can be passed as argument). Then, an email is sent to the user with the link to the respective sponsorship detail page so they can fulfill their application. The command always asks for a manual confirmation before sending the email, but if you want to use it under a cron job, you must use the --no-input flag to skip the confirmation. Here's a sample on how to use it python manage.py check_sponsorship_assets_due_date 7 --no-input.

Here's a sample of the email's content that I got from the terminal:

Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: You have pendings assets for your application
From: sponsors@python.org
To: ber@ber.com
Date: Fri, 21 Jan 2022 19:00:28 -0000
Message-ID: <164279162857.278656.13781464093342473376@macalandia>

Hi,

Your sponsorship application for Bernardo New requires one or more assets with Jan. 22, 2022 as due date.

Please, submit all information in ordeer for PSF to proceed with your application. You can submit your assets via the following link:

https://python.org/users/sponsorships/95/

Thanks,
Python Software Foundation

PS: Sorry for the PR being polluted by commits from #1959, but since this PR also has migrations, I started the work from that branch to avoid complex migrations corrections. I recommend you to review the PR by the commits.