Add global setting to disable/enable unknown sign in email

What does this MR do?

NOTE: I'm marking as WIP again just to prevent merge. To prevent merge conflicts this should be merged after !34102 (merged). Reviews/approvals can continue as normal and I will unmark WIP, point at master, and merge when safe.

Related to #218457 (closed)

Adds a global configuration option to enable/disable the unknown sign in email. Enabled by default.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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