Notify user more regularly

[aidanjungo]

The aim would be to be able to notify Tournesol user more regularly about new features and encourage them to make more comparisons.

• Change privacy policy to describe the newsletter mechanism (opt-out) and few notification email (opt-out)
  • It has been described in the Terms of Service [front] feat: add the page terms of service #1511
• Add term of services front: Add page with General terms and conditions #1452
• Send mail to all user to warn about the change of privacy policy
  • done the 2023-06-29
• Add these options in profile settings when Epic: UI Settings page #1277 is ready
  • Added by [front] feat: add notifications settings to the general settings form #1647
• Add language preferences in settings when Epic: UI Settings page #1277 is ready
  • feat(back, front): users can define their preferred lang for the notifications #1663
• Add these options at registration
  • research and new features notifications are added by [front] feat: notification settings at sign up #1657
  • the language field is still missing
• 3: [back] feat: automatic reminder after account creation #1677
• Check newsletter mechanism (how to update Sendinblue mailling list update?)

This is the main issue, but it could be breakdown into several small issues.

Idea:

• In the reminder mail we send to users, we could add something like:

  The video "XYZ" that your rate well, as been shown to n people last month

[GresilleSiffle]

In the Preferences page we recently added two settings for the email notifications:

• research studies and surveys about the project
• new features

We still need to allow the users to select their preferred language between English en French.

[GresilleSiffle]

The PR #1664 adds the preferred language setting in the Preferences page.