phpbb: Make the username case-insensitve on login again.

phpbb: Make the username case-insensitve on login again.

The username field in the phpbb3 database is case sensitive (i.e., it
has utf_bin collation), whereas in phpbb2 it was case insensitive. This
commit changes the SQL query to make logins case insensitive again,
since a lot of people used a capital letter for registering their
username, but forgot that later on...