Task custom fields label should be required

Problem

• Currently it is possible to save a new field without a label. Label should be required, though.
• See attached image for fixing locales.
• Show a warning pop-up when creating a new custom field:
  EN: Defining and changing field can lead to high resource usage, when saving the field. Please do it with coordination with the translate5 support team.
  DE: Das Definieren und Ändern von Feldern kann zu einem hohen Ressourcenverbrauch führen, wenn das Feld gespeichert wird. Bitte tun Sie dies in Abstimmung mit dem translate5-Supportteam.