R1:77f546c5724f

Allow installs to require email verification

Summary:
Allow installs to require users to verify email addresses before they can use Phabricator. If a user logs in without a verified email address, they're given instructions to verify their address.

This isn't too useful on its own since we don't actually have arbitrary email registration, but the next step is to allow installs to restrict email to only some domains (e.g., @mycompany.com).

Test Plan:
- Verification
- Set verification requirement to…