Add Retro page. Implement models for Sprint, RetroBoard and BoardSticker.
Add simple Retro Board page template
Add icons to sticker lines. Extend summary to 512 chars
Implement autoupdate for retrospective board
Implement functionality to add stickers via Ajax request.
Return error message when no Json object could be decoded from request
Implement basic functionality for voting
Store only one occurance of user id in voters list
Show the list of voters in sticker tooltip
Implement functionality to modify stickers
Implement functionality to limit voting
Add readonly template for inactive retro board