Fix Web UI view layout of ended task

When view the view layout of an ended task (e.g. by clicking on the link provided by a mail notification), buttons like DELEGATE and the task form's buttons are still rendered even though they should not be as the task is completed.

How to reproduce:

• in the Web UI, start a Serial Document Review workflow on a document
• in a terminal, extract current task's document ID with command

  curl -su Administrator:Administrator http://localhost:8080/nuxeo/api/v1/path/default-domain/workspaces/ws1/blank.pdf/@task -H'Content-Type:application/json' |jq -r '.entries[0].id'
  

• in Web UI, choose a participant for task CHOOSE PARTICIPANTS task and click START REVIEW button. Here is the current state of the workflow:
  
• in a new browser tab, navigate to http://localhost:8080/nuxeo/ui/#!/tasks/<TASK_DOC_ID_FROM_CURL_COMMAND>, which shows the view layout of completed task CHOOSE PARTICIPANTS:
  

Expected result: the view layout of a completed task should not allow to click on buttons DELEGATE and task form's buttons (and other things if needed).