Re-add 4.0.18 upgrade step, for those who already upgraded to 4.2.0

The 4.0.18 upgrade step, while not important, was missed by users who
had upgraded from 4.0.x (x < 18) to 4.2.  Re-add the upgrade step as
4.2.1.
Merge branch '4.2/branded-subject-tag-again' into 4.2-trunk