Modernize UI
Modernize the UI by moving to Bootstrap.
Take into account the followin improvements:
- Write the title of grouping boxes (i.e. Burndown, Team's Work, ...) outside of the box. Keep more spacing between the boxes.
 - Remove excessive grid from Burndown.
 - Harmonize links and buttons in the header bar.
 - Clean up Navigation.
 - Introduce user portraits to the User Widget and use it to display user activity. Make the online/offline indication graphical, e.g. by a border of the portraint or make the user entry half-transparent. Black-and-white portraits would also be as possibility but are not suitable if user portraits are already black-and-white.
 - Move System Messages out of the navigation.
 - Introduce a Navigation toggle to the header bar that shows and hides the navigation and lights up if there is activity in the chat.
 - Introduce dedicated handles (distinct from the entity id)
 
Also: iss845?
Estimation
8 Story Points
