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

api/post.go

@@ -58,7 +58,7 @@ func createPost(c *Context, w http.ResponseWriter, r *http.Request) {
 		post.CreateAt = 0
 	}
 
-	rp, err := app.CreatePostAsUser(post, c.TeamId)
+	rp, err := app.CreatePostAsUser(post)
 	if err != nil {
 		c.Err = err
 		return