WIP: Make slack notification use the user's Slack username

What does this MR do?

Improves the Slack integration by using the user's Slack username to properly mention the user in Slack

Are there points in the code the reviewer needs to double check?

• TBD

Why was this MR needed?

• Solve https://gitlab.com/gitlab-org/gitlab-ce/issues/20151

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/20151

Screenshots (if relevant)

• TBD

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together