admin-analytics: fix headings

Created by: erzhtor

Closes https://github.com/sourcegraph/sourcegraph/issues/39939. This PR:

• Changes headers to be H2 instead of H1
• Updates heading dividers to be icon-color

Test plan

• sg start
• Open http://localhost:3080/site-admin/analytics
• Check that all analytics features page headings follow the same styling

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.