Add "Configure SAST" link as primary action for the SAST promo banner

Related to gitlab-design#1730 (closed).

Problem to solve

The promotional security banner that is displayed on the project homepage does a great job introducing SAST, but doesn't give users a direct entry point into the easiest way to configure SAST – the configuration UI

Before finalizing proposal

• verify if this banner is displayed to GitLab core users as well as ultimate and adjust recommendations accordingly

Proposal

• Add a confirmation/primary button to the banner to "Configure SAST". This button should link to the SAST configuration UI
• Change the current "Learn more" link to a default button