Allow authentication from multiple OpenIDs using permission links with
matching email and OpenID prefix.

refs #1722
Support pre-activation by creating a new user record and OpenID
permission link.

refs #1785
refs #1722
Request activation if is_active==false but no user agreements seem to
be required.

refs #1699
Merge branch '1675-preactivation'

closes #1722
closes #1785
Do not set is_active flag for new users. They do that themselves now,
after accepting user agreements.

refs #1699