Follow up: Play all manual jobs in a stage regardless of stage including only manual jobs

Problem

On https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27188 we're relying on the stage status to render the 'Play all manual' button, it's a better approach performance-wise, but it does not offer the best UX experience as the button only appears if all builds in a stage are manual.

Proposal

Would be nice to find a middle ground in which the performance of the PipelineSerializer does not decrease and the 'Play all button' could be displayed if the stage contains some manual builds.

More information

More details on: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27188#note_164345293