Improve ui consistency
- Decline idea of using gray buttons actively in UI
- remove icons from some buttons that already has text
- standardizes margin between buttons
First step in https://gitlab.com/gitlab-org/gitlab-ce/issues/18324
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
First step in https://gitlab.com/gitlab-org/gitlab-ce/issues/18324