use `globalindex.Task` instead of `task.Task` for task overview and dossier task listing.

This PR replaces `task.Task` with `globalindex.Task` for:
 - task overview
 - dossier task listing

It includes a fairly complete rewrite of the task overview view to generally improve the code and readability.

Closes #295.
As we discussed I would include the admin_unit as second parameter for the `by_dossier` method. 

Because the dossier path is not 100% explicit to one dossier.