Rework OGDS and task functionality

This PR contains all the work we did the last weeks to improve the OGDS and task implementation. 

See the [Changelog](https://github.com/4teamwork/opengever.core/blob/7d237091f7f98e54000eef409905e87b11d6d254/docs/HISTORY.txt#L8) or all Issues and PR's wich are marked with the `hutwechsler` label for a small overview.

This PR needs adjustments in https://github.com/4teamwork/opengever.ogds.models/pull/22 and https://github.com/4teamwork/plonetheme.teamraum/pull/253.