[Feature] Refactor work package relations implementation

This branch refactors the work package relations view. The main purpose is to remove code duplication. Therefor, relation handlers are introduced that contain the behavior of the different relation types (regular relations, child relations, parent relations). The relation handlers then inject the specific relations behavior into the common ``work-package-relations-directive.js``.
@hschink I feel that this should not be in the models namespace as it isn't a model. I would suggest making a new namespace, something like viewmodels (I'm open to ideas here).