Apply GitLab UI button styles to buttons in ee/app/views/projects/settings/slacks directory

Issue
#231187

ee/app/views/projects/settings/slacks/edit.html.haml

@@ -15,7 +15,7 @@
           = form.text_field :alias, class: 'form-control', placeholder: @slack_integration.alias, required: true
 
       .footer-block.row-content-block
-        %button.btn.btn-success{ type: 'submit' }
+        %button.btn.gl-button.btn-success{ type: 'submit' }
           .spinner.spinner-light
           %span
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