TaskLists used to be an empty table that linked pages and
their tasks together:
* page.data -> TaskList
* task.task_list -> TaskList

This commit removes the TaskLists entirely. Instead tasks
now belong to the page itself. TaskListPages have no more data
set.

This cleanup removes an intermediate class we never really used anyway.

In general i plan to move away from relying on the polymorphic page
belongs_to data. Instead data belongs_to page and only the page types
it's relevant to have the corresponding has_many associations defined.
This allows for much more flexibility in attaching data to the page.