Very basic user management

Added an user list page and a form to create new users. All users can
create other users.

database/users-functions.sql

@@ -0,0 +1,21 @@
+--
+-- Create a new user
+--
+
+CREATE OR REPLACE FUNCTION users_add( _email TEXT , _salt TEXT , _iters INT , _hash TEXT )
+	RETURNS INT
+	LANGUAGE PLPGSQL
+	STRICT VOLATILE SECURITY INVOKER
+AS $users_add$
+BEGIN
+	INSERT INTO users ( user_email , user_salt , user_iterations , user_hash )
+		VALUES ( _email , _salt , _iters , _hash );
+	RETURN 0;
+EXCEPTION
+	WHEN unique_violation THEN
+		RETURN 1;
+END;
+$users_add$;
+
+REVOKE EXECUTE ON FUNCTION users_add( TEXT , TEXT , INT , TEXT ) FROM PUBLIC;
+GRANT EXECUTE ON FUNCTION users_add( TEXT , TEXT , INT , TEXT) TO :webapp_user;