Creates invalid BitBucket user links for users without username

[tbm]

I got a wrong link like this:

{'body': '**[Original report](https://bitbucket.org/blais/beancount/issue/91) '
         'by Dominik Aumayr (Bitbucket: [Dominik '
         'Aumayr](https://bitbucket.org/Dominik Aumayr), ).**\n'

If you go to the issue at https://bitbucket.org/blais/beancount/issues/91 and hover over his name, you see his name but no user name. Whereas if I go to https://bitbucket.org/blais/beancount/issues/92 and hover over that user, I see a name and a user name.

So it seems some people don't have user names on BitBucket. In that case you shouldn't create a BitBucket link.

[tbm]

Here's a weird example: the first comment at https://bitbucket.org/blais/beancount/issues/100

Blind Gaenger looks like a nick name, Bernd Jünger like the real name, but it's not clear if either is a valid BitBucker username.

[magreenblatt]

Names that include spaces (like "Dominik Aumayr") also don't display correctly as Markup links on Github:

It's probably best to just display these names without the link.