Add GitLab icon to Slack Notifications
What does this MR do?
Adds GitLab's icon to Slack Notifications
Are there points in the code the reviewer needs to double check?
- Not sure what the proper image URL is
Why was this MR needed?
Right now, Slack just uses the default Incoming Webhook icon -- a branded experience is more contextual (and better for branding).
Screenshots (if relevant)
Currently it looks like this: 
Does this MR meet the acceptance criteria?
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Changelog entry added -
Documentation created/updated -
API support added - Tests
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Added for this feature/bug -
All builds are passing
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Conform by the merge request performance guides -
Conform by the style guides -
Branch has no merge conflicts with master(if it does - rebase it please) -
Squashed related commits together