Changed form action link to a relative path name from an absolute path name, which was causing problems on some sites with reverse proxy rules.   See the bug report on the MRBS-general mailing list.

git-svn-id: http://svn.code.sf.net/p/mrbs/code/mrbs/trunk@2838 25f7e783-145e-4f2d-a9f8-dd7182bfe9c7