Propagate logout_reason messages

For ajax requests, horizon.middleware does not currently copy the
logout_reason cookie into newly generated redirect_responses. This
will result in lost logout_reason messages when a user is redirected
to logout.

This patch adds the logout_reason cookie to the list of cookies
copied over from the incoming response to the newly created
outgoing response.

Change-Id: I089eff5627f870c8b3f0b7913693231602a8f9d4
Closes-Bug: #1298074
Merge "Propagate logout_reason messages"