Add `username` and `icon_url` for slack/mattermost integration

Description

In Mattermost/Slack there is no special image and no name for incoming webhook messages.

Proposal

GitLab should set the icon_url to either GitLab logo or to the projects logo. And the username to GitLab.

Links / references

API references:

• https://docs.mattermost.com/developer/webhooks-incoming.html (scroll down)
• https://api.slack.com/incoming-webhooks