Fix "verify email" alert AB test to show only on dotcom

Created by: erzhtor

Closes https://github.com/sourcegraph/sourcegraph/issues/34603. Follow-up https://github.com/sourcegraph/sourcegraph/pull/34605.

Description

This PR:

• Fixes the "verify email" alert AB test to show only on dotcom mode
• Converts GlobalAlerts to function component

Test plan

• connect locally to Postgres and update user_emails.verified_at to NULL to your current local user
• sg start dotcom
• Open https://sourcegraph.test:3443/search?feature-flag-key=ab-email-verification-alert&feature-flag-value=true
• Check that verify email alert shows on top of navbar
• re-run in another mode (ex: sg start enterprise) and make sure the alert doesn't show up

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.