Stacks enter the merge queue together

Queueing a stack from its top used to let the pull requests at the bottom enter the merge queue, and merge, as soon as they were ready, even while the pull request you queued was not. Now the stack waits for all of them: nothing enters until every pull request up to the one you queued meets its queue conditions, and then they enter in order.

• The status comment of each ready pull request says Waiting for the rest of the stack and lists the ones that are not ready yet.
• The Queue this pull request checkbox shows on a stacked pull request as soon as its own conditions are met, so one tick on the top queues the stack.
• If the queue command on the top is cancelled before the stack enters the queue, by new commits, a dequeue or closing it, the pull requests below drop it too: queue the top again. There is nothing to turn on: every stack queued with @mergifyio queue or the checkbox works this way. See Stacked Pull Requests for the details.