Implement missing comment repository, service & controller

[eeebbaandersson]

Currently, the comment package only contains DTOs, Entities, and Mappers.
To follow the architectural pattern of the application, we need to implement the missing repository and service as well.
The final logic can be decided later but a base structure is nice to have in place.

[eeebbaandersson]

While working on this issue the logic for CommentController should be implemented as well.