feat: add allow_newsletter field to account settings

[BryanttV]

Description

This PR adds the allow_newsletter field (Checkbox) in the Account Settings. This allows each user to update the value of the field.

Related Issues

• https://github.com/fccn/nau-technical/issues/279
• https://github.com/eduNEXT/consulting-issues-mapping/issues/10

Testing Instructions

1. Create a Tutor environment in the Redwood version.
2. Install this plugin with the changes in this PR.
3. Create a mount of edx-platform with the changes in this PR.
4. Update the account.redirect_to_microfrontend Waffle Flag. Set Everyone to No. This allows access to the legacy interface.
5. Go to {lms_domain}/account/settings
6. You should see the new checkbox field in the Additional Information section.
7. Try to update the field and verify that the value was updated in the model.

Screenshots

[MaferMazu]

Thanks for this, @BryanttV, this worked as expected ✨

My only suggestion, which is not a blocker, is that although this feature is related to the scheduled nudged emails, it does not have a direct dependency. Therefore, to ensure clarity and that the PRs do not block each other, I recommend that the PR go to redwood.master instead of the nudged branch.

[igobranco]

LGTM ✔️