Simplify test for #query_user_by_name.
Clean up indexes for redis database when removing task.
Throw NotAuthenticated if login fails to lookup username.
Raise NotUnique exception if username is already taken in database.
Use redis database in production.
Add mark as done functionality to web UI.

Extract view logic to TasksView class.
Add task delete feature to web UI.
Fix misplaced subheader in Readme.
Remove UnifinishedTask and CompletedTask.

Having one class with different states is much simpler.
DRY up fetching of tasks from keys in RedisDatabase.
Move nil check to a more appropriate place in RedisDatabase.
Fix error in Readme.
Replace object creation in Task#done with dup.