Mailing list: subscribe/unsubscribe users

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• Close this issue

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

• add model and migration for keeping subscribed users
• add email handler which processes subscription/unsubscription (including confirmation email), it should work only if feature flag (added in https://gitlab.com/gitlab-org/gitlab-ce/issues/57140) is enabled

Model and subscription is partially addressed by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17502 so it can be used.