Add a prefix to `enum source` in `Ci::Pipeline`

Description

Ci::Pipeline model has an enumlator source. It has the following values

enum_with_nil source: {
  unknown: nil,
  push: 1,
  web: 2,
  trigger: 3,
  source_schedule: 4,
  api: 5,
  external: 6
}

We renamed schedule to source_schedule in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21767 because it was conflicted with state machine's schedule!. We should refacor others as well to make it consistent.