feat(components): moving subnav property to nav-item component

[owilliams320]

Description

moving the required subNav property from the list component scss to the cv-navigation-item component. This makes the styling for the subnav items to be more robust when wrapped with an other elements like an a tag

Test Steps

• nx serve component
• then open the localhost
• finally test that visually nothing has changed

General Tests for Every PR

• npm run start still works.
• npm run lint passes.
• npm run stylelint passes.
• npm test passes and code coverage is not lower.
• npm run build still works.

Screenshots or link to StackBlitz/Plunker