Implement some post endpoints for APIv4 (#5353)

* Implement POST /posts endpoint for APIv4

* Implement GET /channels/{channel_id}/posts endpoint for APIv4

* Implement GET /posts/{post_id} endpoint for APIv4

* Implement GET /posts/{post_id}/thread endpoint for APIv4

* Skip team get if it's a DM channel in handlePostEvents

model/post_list.go

@@ -75,7 +75,12 @@ func (o *PostList) Etag() string {
 		}
 	}
 
-	return Etag(id, t)
+	orderId := ""
+	if len(o.Order) > 0 {
+		orderId = o.Order[0]
+	}
+
+	return Etag(orderId, id, t)
 }
 
 func (o *PostList) IsChannelId(channelId string) bool {