Add option to use the username in Slack messages instead of the full name

Description

In bigger teams employees with the same first name are very common. At least in our company.
That leads to many false mentions in Gitlab Mattermost. I expect the issue is the same as in Slack.

As example:

Sebastian Poxhofer opened merge request !52 in Gitlab.org/Gitlab Community Edition: did something.

In this case every Sebastian in our team would be notified.

Proposal

Add a option to the Slack service to use the username instead of the full name .

secustor opened merge request !52 in Gitlab.org/Gitlab Community Edition: did something.

This issue could be solved at the same time: #22735 (moved)

~"feature proposal"