Rewrite yellowfin cookie expiry time from 12h to 31d

Despite setting the session expiry time in yellowfin's
web.xml, the cookies that its login function returns
still expire after 12 hours. Since we are rewriting their
data anyway, it is easy for us to change their cookie
expiry times, which we do.
The constant 31 days inside yellowfin.go should be kept
in sync with the constant inside yellowfin's web.xml