Add option to send GitLab username or full name with Slack notifications

Zendesk: https://gitlab.zendesk.com/agent/tickets/83485

Customers requests an option to choose between sending a user's full name or username with Slack notifications. Their LDAP has mostly unrecognizable usernames such as an employee ID. This is fairly common among our customers so I expect others would appreciate this feature as well.

An MR from a year ago changed the default from using full name to username - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6624

cc/ @jramsay Are project services in your court?