chart-diff: does not work correctly for draft charts

[larsyencken]

Problem

The chart diff flow hasn't been designed for draft charts, so lingering draft charts can cause it to crash.

Expected behaviour

Draft charts should also be queued for approval, like other charts.

Workaround

We currently ignore draft charts in the chart diff.

[larsyencken]

We triaged this as "important" since you might expect that your work as draft charts would be shipped, but instead you could lose that work after merging.