If the user's preferences are for the site default screen, display a
welcome bar in the header with a link to the new screen documentation.
Immediately update the user's preference to be either the old screen,
or the new screen to prevent the bar from displaying again.
For anonymous users save a "gerrit_cs2" cookie for 7 days with the
value of "0" (old screen) or "1" (new screen) when the user clicks
a link to dismiss the welcome bar.
Change-Id: Idc371732bbfa160e47075b8d01bc877a46d1b723
ee772c5252