Implement User Comments with simple crud functionality

[KonradUdoHannes]

As a first implementation we could implement comments as a standalone entity. For integration in the frontend a separate issue should address needed relationships with comments and other entities such as specific projects etc.

Comments should include:

• the comment text (allowed length?)
• the user who wrote the comment
• the time the comment was posted
• the time the comment was updated

For CRUD the following constraints should apply:

1. Every user can create a comment by default
2. Users can only update or delete their own comments
3. Users have access to comments that
   • they wrote themselves
   • that are associated with an entity (for instance a project) that they have access to

As relationships with other entities are not part of this issue, point 3. should also not be part in the first implementation. For the initial implementation its fine if comments can be accessed by all authenticated users.