jQuery 3 checks for any styles before dimension so
a value typical of the browser defaults needs to
be provided when mocking them.
This helps the mocking of `$.height` by providing
a default style for `height`.
The button shouldn't change its vertical position
when the state changes. The text confirming the
copy is just one line so setting height for both
based on the API key, which can run to 2 lines
makes sense.
Explained in this PR:
https://github.com/alphagov/notifications-admin/pull/2428
