Store flash messages in a different session variable

This should solve the problem we've seen in QA when testing the new login/logout feature.

From my analysis this is due to the difference in how Rails 3 and Rails 4 store flash messages in the session. Rails 4 can deal with the format used by Rails 3 but not the other way around, hence the whitescreens/500s we were seeing intermittently.

This is a monkey patch and has no testing but seems to do the trick. Any feedback would be more than welcome.