Mailing list: add model and UI for mailing list settings

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

• adds model for mailing list functionality
• adds basic UI for mailing list setting, I expect that for first iteration we will use email addresses from gitlab.yml so there should not be much to set except enabling/disabling the ML
• adds feature flag for mailing list functionality

Mostly already done by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17502