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 true.
  • Tried to use Phabricator with an unverified account, was told to verify.
  • Tried to use Conduit, was given a verification error.
  • Verified account, used Phabricator.
  • Unverified account, reset password, verified implicit verification, used Phabricator.
• People Admin Interface
  • Viewed as admin. Clicked "Administrate User".
  • Viewed as non-admin
• Sanity Checks
  • Used Conduit normally from web/CLI with a verified account.
  • Logged in/out.
  • Sent password reset email.
  • Created a new user.
  • Logged in with an unverified user but with the configuration set to off.

Reviewers: btrahan, vrana, jungejason

Reviewed By: btrahan

CC: aran, csilvers

Maniphest Tasks: T1184

Differential Revision: https://secure.phabricator.com/D2520