Use sql tasks in dossier task-overview.
Extract `OpengeverTab` to its own module.

To avoid circular imports.
Fix wrong upgrade id.
Fix upgrade version check.
Add containing_subdossier to task globalindex.
Use `GlobalTaskListingTab` to render dossier task lists.
Add plone.api to the dependencies.
Also add a text column to `globalindex.Task`.
Remove useless mocktestcase.
Refactor task-overview, use SQL objects when possible.
Mark method as to be removed, reword method name.
Display icon on task overview when task is overdue.

Closes #333.
Move task link method to `globalindex.Task`.
Make admin_unit a required parameter for `TaskQuery.by_dossier`.
Merge pull request #341 from 4teamwork/deif-use-sql-task

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