Refactor issuable description and metadata form sections
Continuation of https://gitlab.com/gitlab-org/gitlab-ce/issues/23864.
Brother of gitlab-org/gitlab-ee!916.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Continuation of https://gitlab.com/gitlab-org/gitlab-ce/issues/23864.
Brother of gitlab-org/gitlab-ee!916.