#57140: add mailing list schema and models

What does this MR do?

Create the model, the schema changes, the feature flag and the administration UI for the mailing list.

• database schema
• ruby models
• instance settings
• project settings
• feature flag

What are the relevant issue numbers?

Subtask of https://gitlab.com/gitlab-org/gitlab-ce/issues/4272#note_115526454

Implements #57140 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Screenshots

Instance setting

Project setting