Create project Entity

Closes #266 (closed).

I created the entities for JobHolder, Project and ProjectJobOffers. To keep in mind the TAM planning document did not have the id field within the super-class: JobHolder, but it is not possible to create an entity without an id (the other option would be to mark the JobHolder class with a @MappedSuperclass, but this makes it impossible to use JobHolder as a reference, let's say in JobOffers).

I left a couple TODOs in the code, since the other subtasks depend on these entities a lot, but they are not final.