[MM-44263] Only include user posts in post count (#20250)

* Fetch users only posts

* Explicitly validate posts count before testing usage

model/post.go

@@ -280,6 +280,7 @@ type PostCountOptions struct {
 	MustHaveFile    bool
 	MustHaveHashtag bool
 	ExcludeDeleted  bool
+	UsersPostsOnly  bool
 }
 
 func (o *Post) Etag() string {