User notifications are defined as
 * notes;
 * pushed to user Inbox or email through classic notification process;
 * not linked to any document, meaning model and res_id are void;

When pushed in user's Inbox they are currently displayed with a gray
background like all notes. However UX experts think it should use a white
background to be easier to spot in chatter. This commit makes it work.
From now on discussion and user's notifications are displayed using a white
background. This commit is related to task ID 1829872.