Sync up auth demo plugin with what's actually used

The newer version is at least used by the pgeu system and by the
new commitfest one, and has some fallback for the duplicate-email
issue.
Implement user searching in the community auth system

This lets downstream systems securely search for users that are in
the system, so they can populate their local database with users
before they have logged in if necessary. This can be used for example
for the commitfest management system to be able to flag users as
authors and reviewers even before they have logged in.