Add basic user notification system

Add a notification system that will show notifications of interesting events to users. The initial implementation should work for a single event, with other events and functionality as later enhancements.

When a project-version changes from 100% translated to less than 100% translated for a locale, a notification should be generated for each translator who has contributed a translation to the project-version in the locale. The notification should have a message in the spirit of "Some translation is needed on this project, you may want to take a look at it," and include information about which project-version and locale it is about.

The notification will stay in the user's inbox until they have marked it as read. After it is read, the notification will become eligible for deletion 30 days after the event about which it notifies.

1. User Interface

Add “All Notifications”/”Notifications” tab on the dashboard.

• Tab shows the number of notifications on its icon/name

• Explicit mark-as-read operation is available

• Tab can be bookmarked

• Read notifications are still shown in the list, but visually different and do not contribute to the number on the icon