Update the header of the admin Email Style page to be more consistent with the rest of the admin UI.
The HTML/CSS tabs have also been updated, which required an extra check being added to the NavItem component.
1. Use `this.` instead of `{{action}}` where applicable
2. Use `{{fn}}` instead of `@actionParam` where applicable
3. Use non-`@` versions of class/type/tabindex/aria-controls/aria-expanded
4. Remove `btn` class (it's added automatically to all DButtons)
5. Remove `type="button"` (it's the default)
6. Use `concat-class` helper
