[backport] fix focus on open lightbox

app/coffee/modules/common/lightboxes.coffee

@@ -44,9 +44,8 @@ class LightboxService extends taiga.Service
 
         @animationFrame.add ->
             $el.addClass("open")
-
-        @animationFrame.add ->
-            $el.find('input,textarea').first().focus()
+            $el.one "transitionend", =>
+              $el.find('input,textarea').first().focus()
 
         @animationFrame.add =>
             lightboxContent.show()