Rewrite ad-hoc @ tracaccountmention to github @ mention

[mkoeppe]

Some Trac tickets use the ad-hoc syntax for mentioning another user, for example @gh-kliem or @mkoeppe.

Currently we escape the @ so that it does not show up as a github mention.
But when we see @ followed by a valid Trac account, we can rewrite it to an actual github mention.

[mkoeppe]

Fixed by #56