Implement Project Life cycle functionality scaffolding

[KonradUdoHannes]

One of the main tasks of the engage is to manage the project life cycle. This is a complex tasks but we can be begin by setting up the life cycle transitions without any additional project data and project data validation.

A minimal setup would require

• a project model with a project_phase attribute
• a representation of the allowed project phases (most likely an enum)
• a representation of the allowed project phase transitions. For instance it might always be allowed to go to the next phase or abort entirely. It might also be allowed to skip certain phases in a specific project. A possible reprentation could be a mapping from current phase to a list of allowed next phases.
• a PATCH endpoint that allows updating the project phase the transition is valid.

To make testing more convenient this issue could be done after #22