Make placeholder highlighting align at all size
Make placeholder highlighting align at all size

Because the placeholder highlighting was defined in pixels it got
slightly out of line when it was used at larger type sizes, eg inside a
heading.

By using `em`s it will scale with the size of the type.

alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin