Bug 1070046: MySQL has an optional server config option, "MAX_JOIN_SIZE",
which throws an error if you try to run a SQL query that MySQL's strategizer
thinks will require it to evaluate more than MAX_JOIN_SIZE rows. This is
intended to prevent the user from accidentally running giant queries that
will never finish, but some Mahara queries (which are large but will finish)
can trip it. Adding SQL_BIG_SELECTS=1 tels it that our queries are *not*
accidentally large.

Change-Id: I6db4699ea765d3213d13eb93b8de098914db24e0