update with respect to welcome message api changes in SiteMailingList. $send_welcome is a class var so that subsclasses  can change it if needed.
update with respect to welcome message api changes in SiteMailingList. $send_welcome is a class var so that subsclasses 
can change it if needed.


svn commit r51248

nburka/site,silverorange/site,silverorange/site,nburka/site