Parent issue is #1122198: Filter to make linking to drupal.org nodes faster/easier

Add support for .user to dopl. User names with spaces in them is an issue. I don't think we can use user id, as most people will have to look them up first.

Do we even want this functionality? If so, would it link to the user page, or directly to the contact page. I'm imagining the use case is "Try talking to davidhernandez.user about that" or "If you want help with that, let me know - davidhernandez.user". Are we afraid this will increase the likelihood of unwanted contacts, and harassment/spam?