Add developer documentation about working with sent emails and previews
Been meaning to document letter opener for a long time, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14161 finally spurred me!
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Been meaning to document letter opener for a long time, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14161 finally spurred me!