add support for "Promote & Push" button in Ship-it

[lundjordan]

This would help the devedition and beta workflow for relman. It was called out at today's release postmortem

Currently these actions are separate. It's useful to have them separate in the b1 case but for b2+, they should be triggered at the same time in shipit. The problem is, you must wait for the decision task to complete on the promote graph before being able to trigger the push graph. And it's inconsistent in when relman triggers the push graph, if they remember to at all.

option 1) shipit determines if the version is b2+ and instead of having two separate phases, it has just a "Promote & Push" phase. Behind the scenes, this triggers both graphs one after the other. Each graph once they exist is clickable from shipit UI as it is today.

option 2) shipit continues to show "Promote" and "Push" as separate buttons but there is a third option (above the pipeline UI?) that allows you to do both at the same time.

option 3) shipit and taskgraph has no concept of a separate push graph phase for b2+. So only one graph exists for both promote and push.