Add the assignee to Slack messages

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Description

We are using right now Gitlab, Gitlab Mattermost and MantisBT (Support) in our organisation.
In our case, we are using Mattermost for centralizing all informations at one place and obviously for communication. Because of this, the users get notified if new issues, merge request or similar are created.
The problem is, only the creator of the issue is referenced. So the employee, which is assigned to the work, is not notified and doesn't get a 'mentioned' event.

Example:

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

Proposal

Add a for <username>,if the assignee is set and he is not the creator.

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

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

This issue could be solved at the same time: gitlab-ce#22737

~"feature proposal"

Edited by 🤖 GitLab Bot 🤖