Remove verified attribute from posters.

This attribute will be added into organizations instead -- any poster
that belongs to a verified organization is itself verified.
Add verified attribute to organizations
Unverified organizations cannot log in.

Upon signup, the verified attribute is set to false. Until this
attribute is set to true, the organization cannot sign in (and thus
cannot add any posters or do any other privileged actions).
Admins can verify organizations through edit page.

A 'Verify' button now appears on an organization edit page when the user
is an admin and the organization has not yet been verified. Once
clicked, the organization is verified and can sign in.