Implement a history of resolution versions

[scattenlaeufer]

This will be realized by linking different versions of resolutions.

Requirements are:

• [x] Link a Resolution to it's parent and child resolutions
• [x] A Resolution can have more than one parent or child
• [x] ResolutionListView should only display the latest version of a Resolution
• [ ] Amendments need to be implemented
  • [ ] An amendment is a new Resolution, linked to the original one
  • [ ] An amendment isn't shown in ResolutionListView, but on the ResolutionDetailView of the Resolution it amends.
• [ ] Implement a way to unify multiple Resolutions to one single motion.