Pipeline status Slack notifications don't show the project's avatar image in the footer

Summary

The project's avatar image is not shown in the footer of Slack notification messages for pipeline failures:

When a project has an avatar image, this is what the notification should look like:

Steps to reproduce

Take a look at any of the pipeline failure notifications in our (internal) #development Slack channel.

Output of checks

This bug happens on GitLab.com.

Possible fixes

This call to project.avatar_url should include only_path: false as an argument.