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

api4/context.go

@@ -375,6 +375,17 @@ func (c *Context) RequireUsername() *Context {
 	return c
 }
 
+func (c *Context) RequirePostId() *Context {
+	if c.Err != nil {
+		return c
+	}
+
+	if len(c.Params.PostId) != 26 {
+		c.SetInvalidUrlParam("post_id")
+	}
+	return c
+}
+
 func (c *Context) RequireEmail() *Context {
 	if c.Err != nil {
 		return c