Add ConversationPartController with index endpoint and scope

[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

To be able to fetch ConversationPart records from the client asynchronously, we need a working index endpoint

Subtasks

• [ ] Implement Policy.ConversationPart.scope/2
  • scopes to ConversationPart records belonging to Conversation records defined by Policy.Conversation.scope/2

# Example implementation

def scope(query, current_user) do
  conversation_scope = Conversation |> Policy.Conversation.scope(current_user)

  query 
  |> join(:right, [cp], c in subquery(conversation_scope), cp.conversation_id == c.id)
end

• [ ] add tests for Policy.ConversationPart.scope
• [ ] Implement :index endpoint
  • needs to support coalesced id filter - we will be fetching conversation.conversationParts in the client
  • might need to support filtering by conversation_id, but not sure yet
• [ ] add tests for :index endpoint