Commenting

[rubenhorn]

⚠ Confirm with design and frontend if the specification for this feature is ok!

Tasks:

• Comment threads are fetched by crowdaction(id) (one thread per crowdaction)
• Comments can be deleted* by the author (return placeholder for deleted comment in thread)
• Comments can be edited (?) (indicate with additional boolean field in response item)
• Pagination
• API documentation is updated (!)
• Spam protection e.g. rate limit (How would we implement this? Investigate!)

*Implement using flag, so comments can be "un-deleted".

[rubenhorn]

Branch already started. Please continue on #96.
⚠ Make sure to merge development first to update the branch!