Merge branch 'fix-haml-slack-button' into 'master'

Fix HAML in ee/app/views/projects/services/gitlab_slack_application/_slack_button.html.haml

See merge request gitlab-org/gitlab!58545

ee/app/views/projects/services/gitlab_slack_application/_slack_button.html.haml

 %a#slack-button{ href: add_to_slack_link(project, slack_app_id) }
-  %img{ alt:"Add to Slack", height: "40", src: "https://platform.slack-edge.com/img/add_to_slack.png", srcset: "https://platform.slack-edge.com/img/add_to_slack.png 1x, https://platform.slack-edge.com/img/add_to_slack@2x.png 2x", width: "139" }
+  %img{ alt: "Add to Slack", height: "40", src: "https://platform.slack-edge.com/img/add_to_slack.png", srcset: "https://platform.slack-edge.com/img/add_to_slack.png 1x, https://platform.slack-edge.com/img/add_to_slack@2x.png 2x", width: "139" }