DEV: Clean up selectors on backup modal (#9114)

app/assets/javascripts/admin/templates/modal/admin-start-backup.hbs

@@ -1,12 +1,14 @@
 {{#d-modal-body title="admin.backups.operations.backup.confirm"}}
   {{d-button
-    class="btn-primary"
+    class="btn-primary backup-with-uploads"
     action=(action "startBackupWithUploads")
     label="yes_value"}}
   {{d-button
+    class="backup-no-uploads"
     action=(action "startBackupWithoutUploads")
     label="admin.backups.operations.backup.without_uploads"}}
   {{d-button
+    class="btn-default"
     action=(action "cancel")
     label="no_value"}}
 {{/d-modal-body}}