Handle skipped stages and parallels in the Pipeline Graph

Context
The Pipeline team have added the ability to "skip" stages in the Pipeline (JENKINS-37781).

Scope

• Skipped stages are shown as not built
• Declarative Pipeline will not provide flow nodes for skipped parallels so we can always assume that there won't be any skipped parallels.
• If the first stage is skipped then the pipeline should start from a "Start" node.
• If the last stage is skipped then the pipeline should end with a "End" node.
• Ensure that the editors start/end nodes match the style in this mockup

Mockup