Add a page to display completed tasks and add the completion date column for tasks
Add kiosk mode, public board access with read-only and auto-refresh
Update for Polish translations.

I've added missing translations and update a few exiting (in Polish we do not use space before and after ':' and '?').
A few more updates in translations.

There is no context for strings so sometime it is hard to guess what would be the best translation.
Add new contributor
Add helpers for the user session and fix bug edit_user admin checkbox
Display an error message when there is no column for a project
Add the possibility to change the assignee directly from the board
Remove project dropdown from task edit
Change mouse cursor for draggable items
Added missing translations
Fixed timezone warning by adding timezone config option