Set max width on legacy gadgets

Legacy gadgets must be <= 725px wide or else some visual elements are displayed incorrectly (e.g. color picker).
Possibly we need other style defaults, `overflow:hidden;` etc., without which legacy gadgets will get into trouble, but I'm not sure what other CSS to add here.
This is not valid CSS, it's missing something like `versal-legacy-launcher {`.

Also please add a test for this behaviour.