Add pinned posts (#4217)

store/sql_post_store.go

@@ -64,6 +64,7 @@ func (s SqlPostStore) CreateIndexesIfNotExists() {
 	s.CreateIndexIfNotExists("idx_posts_channel_id", "Posts", "ChannelId")
 	s.CreateIndexIfNotExists("idx_posts_root_id", "Posts", "RootId")
 	s.CreateIndexIfNotExists("idx_posts_user_id", "Posts", "UserId")
+	s.CreateIndexIfNotExists("idx_posts_is_pinned", "Posts", "IsPinned")
 
 	s.CreateFullTextIndexIfNotExists("idx_posts_message_txt", "Posts", "Message")
 	s.CreateFullTextIndexIfNotExists("idx_posts_hashtags_txt", "Posts", "Hashtags")