[back] feat: add mgmt command to notify users without contribution

[GresilleSiffle]

related issues #1677

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Description

This PR adds a new management command that sends email notifications to users that haven't contributed since their registration. The Ansible recipe has been updated to deploy a new service running this command on a daily basis.

To receive an email notification the user:

• must have created his/her account ten days ago
• must not have made any comparison after the registration day

To run the command

python manage.py remind_registrations

Preview

Email received by users without comparison:

Email received by users with comparisons:

to-do

mgmt command

• notify users created 10 days ago who have made comparisons only the first day
• notify users created 10 days ago who have not yet made a comparison
• clearly say that this message is automatic
• translate the template signup_contrib
  • the email title
  • the email body txt
  • the email body html
• translate the template no_contrib
  • the email title
  • the email body txt
  • the email body html
• use the user's preferred language
• add new tests
• do not hardcode the extension URLs

front end

• update the terms of service
  • addressed by b5becc1

infra

• create the systemd timer

Checklist

• I added the related issue(s) id in the related issues section (if any)
  • if not, delete the related issues section
• I described my changes and my decisions in the PR description
• I read the development guidelines of the CONTRIBUTING.md
• The tests pass and have been updated if relevant
• The code quality check pass

[GresilleSiffle]

to-do

• reword the video suggestion links