Allow email notifications to be disabled for all users of a group

What does this MR do?

This MR takes over from the original MR

Allows owners of a group or project to disable all email notifications on a per-group or per-project basis, regardless of the users settings.

Disabling for a group cascades down - all projects and sub-groups will have their email notifications disabled.

EE version at https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15026

UI changes for enabling/disabling are here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30961

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created (to be documented in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30961)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Original issue: #50020 (moved)