Add ConversationPartController with show endpoint and policy

[begedin]

Problem

See #1286 for an explanation of how a Message system works.

Once we have created a Message and it's associated Conversation, any side of the conversation can post a reply.

Each of these replies then creates a ConversationPart containing its contents.

Since we will be fetching ConversationPart records asynchronously, we need to have a show endpoint.

Subtasks

• implement Policy.ConversationPart.show?
  • current_user is authorized if current_user.id == record.author_id
  • current_user is authorized if current_user.id == record.conversation.user_id
  • current_user is authorized if current_user.id == record.conversation.message.author_id
  • current_user is authorized if admin or higher on record.conversation.message.project
• write tests for Policy.ConversationPart.show?
• implement ConversationPartController :show endpoint
  • fetches by id
  • authorizes using policy
• write tests for :show endpoint