batches: warn the user when selecting all changeset specs on the preview page can't actually select them all

Created by: LawnGnome

This is split from #22912 (closed):

We've had considerable discussion of what to do when the user selects all changesets, but some (presumably off screen) changesets are unselectable due to their current state. The desired option in the longer term is to inform the user of this situation, most likely through a new banner, and allow them to filter down to the affected changesets.

This can be seen in the design for #22912, specifically in the third screen.