Problem

I'm not 100% positive on this, but we likely need a :show endpoint in ConversationController

Subtasks

[ ] Implement show? policy
- current_user is authorized if conversation.user_id == current_user.id
- current_user is also authorized if conversation.message.author_id == current_user.id
- current_user is also authorized if they are admin or higher on conversation.message.project
[ ] write tests for policy
[ ] Implement show endpoint in ConversationController
- fetches by id
- authorizes using policy
[ ] write tests for controller