Allow restriction of permitted email domains

Summary:
Allow allowed email addresses to be restricted to certain domains. This implies email must be verified.

This probably isn't QUITE ready for prime-time without a few other tweaks (better administrative tools, notably) but we're nearly there.

Test Plan:

• With no restrictions:
  • Registered with OAuth
  • Created an account with accountadmin
  • Added an email
• With restrictions:
  • Tried to OAuth register with a restricted address, was prompted to provide a valid one.
  • Tried to OAuth register with a valid address, worked fine.
  • Tried to accountadmin a restricted address, got blocked.
  • Tried to accountadmin a valid address, worked fine.
  • Tried to add a restricted address, blocked.
  • Tried to add a valid address, worked fine.
  • Created a user with People with an invalid address, got blocked.
  • Created a user with People with a valid address, worked fine.

Reviewers: btrahan, csilvers

Reviewed By: csilvers

CC: aran, joe, csilvers

Maniphest Tasks: T1184

Differential Revision: https://secure.phabricator.com/D2581