Add in-app notifications
We need to display notifications to the users.
We should have:
- a specific widget to display notifications (at the bottom?).
- (optionally) we may want to display those messages as a tray popup.
- Different styles to identify priority (e.g. low, normal, high)
- message hiding possibilities: on click, on event, on X time, a close button
- html tag adding for links, convert plain link to clickable link
Note: we already implemented a simple message ui on #7552 (closed).
(from redmine: created on 2015-11-12, relates #7602 (closed))