Year 3000 problem while displaying thread pins.

Currently, the threads pins assume the thread was made in year 3000. This is a problem, as software is often used long time after it was developed (I know, you think that that there are 986 years, and surely Discourse will be replaced by something by then, but this is a serious problem :laughing:).

However, it doesn't appear that there are huge y3k problems. The only one I found is in topic_query_sql.rb, to be exact, the problem is that pins appear after posts made in year 3000. This pull request fixes it by using PostgreSQL's `infinity` special date value instead.