Clean up schema for broadcast_messages
From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):
- Missing constraint: starts_at NOT NULL
- Missing constraint: ends_at NOT NULL
- Missing constraint: message_html NOT NULL
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):
assigned to @yorickpeterse
changed milestone to %9.5
mentioned in commit 77ca7c08
mentioned in commit a5c8a527
closed via merge request !13429 (merged)
mentioned in commit c19ea197
mentioned in commit 345a304a
mentioned in commit 52cd6fa3
mentioned in commit 3ef2fb2b
mentioned in merge request !14785