Add verify email notification alert AB test

Created by: erzhtor

Closes https://github.com/sourcegraph/sourcegraph/issues/34603

Description

This PR:

• Adds email verification global alert experiment
  • All non-verified emails for the current user will show up on the global alert place (on top), which stays there on all web pages

Screenshots

Description	Screenshot
single non-verified email
multiple non-verified emails

Event logs

• ab-email-verification-alert=true|false event/user property
• ViewUserSettingsEmails event. Sent whenever user visits email settings page
• UserEmailAddressVerificationResent event. Sent whenever user clicks "resend verification email" on email settings page

Test plan

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.