Make the snackbar transitions animated

The snackbar should "minimize" itself as it's going away, using a css transition for width/height (probably cubic bezier). So, instead of making it just disappear, it should transition to a small width (say 5pxX5px or something) and THEN disappear.

Locate the snackbar implementation and its current dismissal path; the issue names no file or test. Verify completion by observing that dismissal shrinks the snackbar toward roughly 5px by 5px with a CSS transition before it disappears.

Written by the indexing model from the issue text.