Continued with cellphone number edit: (please cherry-pick)

Continued with cellphone number edit: (please cherry-pick)
- no, the data cannot be set in the object, it needs to be set in controller (backing bean, don't forget this)
- made both fields required
- if nothing has changed, nothing needs to be updated (check both, mobile provider and number)
- added missing i18n strings
- renamed form id form_edit_phone -> form_edit_cellphone