Replace mentions of Slack usernames

[waldyrious]

Currently direct mentions of a Slack user is shown in Discourse as the user code (unlinked, too), which is not useful and can be confusing if several mentions are made in the post.

Here's an example, which is rendered on Discourse like this:

Thanks <@UGR3910CQ>

It would be ideal if a mapping could be made between the Slack and Discourse usernames, but just replacing the mentions by the username would be a positive change.