ChangeScreen2: Display a welcome bar to new users

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
Set up Release Notes for 2.8.1

Change-Id: Ia8e1680a1facf85f60ce4f7cdf50a4e3c567f93d
Fix broken link in 2.7 release notes

Change-Id: I64ccf7d9795b55040670815688261e4cb35b2e41
Merge branch 'stable-2.7' into stable-2.8

* stable-2.7:
  Fix broken link in 2.7 release notes

Change-Id: Iaf92b8ea36f3b2d24aa4a9f8a793d1f09ebea23c
Merge branch 'stable-2.8'

* stable-2.8:
  Fix broken link in 2.7 release notes
  Set up Release Notes for 2.8.1

Change-Id: Ib12d022953e15ef6091f9ee76996e9f49f3abee7
Merge "ChangeScreen2: Display a welcome bar to new users"