Allow plugins to contribute messages to the UI during initial page
load. Gerrit displays the messages in a butter bar at the top of
the page and allows users to hide them with the "Dismiss" button.
Messages are hidden per-browser using a cookie named after the
message id, set to expire at a date supplied by the server. After
this date the same message could redisplay if the server sends the
same message again.
Change-Id: I0bcca845f501cbeb8c31356fff398c3adb43099a
dfbe6d6ead