Added timestart and timeend field to user_teachers table

(in preparation for enrollment architecture)

lib/db/postgres7.sql

@@ -264,6 +264,8 @@ CREATE TABLE prefix_user_teachers (
    authority integer NOT NULL default '3',
    role varchar(40) NOT NULL default '',
    editall integer NOT NULL default '1',
+   timestart integer NOT NULL default '0',
+   timeend integer NOT NULL default '0',
    timemodified integer NOT NULL default '0',
    timeaccess integer NOT NULL default '0'
 );