Mattermost slash commands
Description
Add support Mattermost for ChatOps.
Proposal
https://gitlab.com/gitlab-org/gitlab-ce/issues/22540#note_17810927
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Add support Mattermost for ChatOps.
https://gitlab.com/gitlab-org/gitlab-ce/issues/22540#note_17810927