batches: supply tooltip reason if changeset not selectable

Created by: courier-new

I couldn't help myself; since we "have more time" anyway to get this wrapped up, I decided to improve the UX for when a preview changeset isn't selectable. There's a lot of potential reasons we wouldn't be able to modify the publication state for a given preview changeset, but the disabled checkbox doesn't communicate all those reasons. @rrhyne + the rest of the team in attendance of today's design session weren't necessarily in love with the next best idea of just telling users to search the docs for the FAQ entry I'm also adding about this, so for marginally more effort, the utility method we were using to identify publishable changeset specs can now provide the exact reason it deemed a preview changeset unworthy unselectable. 😉

Here's some examples of how it manifests:

Wording suggestions welcome.