The flow displayer misaligns a change's media and displays none when the change has no screenshot

The flow change displayer lays each change out as a two-column row: the change's media on the left, its title and summary on the right. It is reachable from the displayChanges macro, and from the Report page with displayer=flow.

To reproduce, on a wiki with the demo XAR installed, open ReleaseNotes.Code.Report with ?action=report&displayer=flow and every filter left empty. Two things are wrong with the media column.

The media is not top-aligned with the text

The screenshot on the left is not aligned with the top of the title and summary on the right, so the two halves of a change do not read as belonging to each other, and a screenshot can end up sitting beside the text of the following change. In that report, the change titled Templates from JARs shows an image next to it that is not aligned with it - and that is not even its own, since that change has no screenshot at all.

The media of a change must be top-aligned with that change's title, and must never be able to be read as illustrating the change next to it.

A change with no screenshot gets no media at all

flow calls #displayScreenshots($changeObject, true, false), so a change that has no screenshot gets an empty left column: the row's text sits beside a blank half-width gap. In that report, 4 of the 15 changes are in that case (Templates from JARs, Velocity and ScriptContext synchronization, Author Executor, Filter Stream annotations).

The grid displayer already does the right thing here: it passes $alwaysDisplayScreenshots as true, which displays the change's screenshots when it has any, its video when it has no screenshot, and the no_image_thumb.png placeholder when it has neither. flow should use that same screenshot/video algorithm.