Security Dashboard Component: Replace main-tag with article-tag

Summary

Currently the security dashboard component (ee/app/assets/javascripts/security_dashboard/components/app.vue) is using a <main> tag to wrap some of its content.

After a discussion (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15050#note_210403348) it was decided to replace the <main> tag with an <article> tag.

Improvements

This will make the component more re-usable and will prevent the possibility of introducing an invalid or non accessible markup structure, i.e. if ever the component gets mounted on a page that already contains a main tag it will cause issues with assistive devices.

Risks

No risks.

Involved components

• ee/app/assets/javascripts/security_dashboard/components/app.vue