Make sure only one click is needed to discard the admin messages

1) Go to Admin Tab / Monitoring / Administrator Message / Edit Status

2) Set an administrator message, set it to Active and save

Current behavior:
When changing the main tab (Home, Workspace, Search, Admin), discarding the message once is not enough. It has to be discarded in all 4 tabs.
(a cookie is set in each page)

Improved behavior:
User should click only once
(a cookie could be set at the nuxeo root instead, thus changing tabs will keep the message discarded).