Getting rid of the completedList and completedIds

Instead only using the 'completed' attribute of the todo task. This fixes some issues were a task was at the same time in the todoIds and completedIds as described in #73.

This change requires an update of all the users in the database. The user.completedIds have to be merged into user.todoIds and can then be removed. I don't know where one would do that. Is there something like database migrations? If someone can give me a hint I'd be happy to write the update script.