Send Gitlab Logo as icon url to Slack

What does this MR do?

Adds Gitlab Logo to Slack webhook.

Why was this MR needed?

It adds branding to, useful in a flow of different notifications.

Screenshots (if relevant)

https://api.slack.com/docs/messages/builder?msg=%7B%22text%22%3A%22I%20am%20a%20test%20message%20http%3A%2F%2Fslack.com%22%2C%22username%22%3A%22Gitlab%22%2C%22icon_url%22%3A%22https%3A%2F%2Fgitlab.com%2Fgitlab-com%2Fgitlab-artwork%2Fraw%2Fmaster%2Flogo%2Flogo-square.png%22%2C%22attachments%22%3A%5B%7B%22text%22%3A%22And%20here%E2%80%99s%20an%20attachment!%22%7D%5D%7D

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

#22268 (moved)