[FIX] fields: apply user timezone in display name for models using a datetime as name

For models using a datetime field as name (hr.attendance for instance), the user timezone wasn't applied in the display name.
Therefore, in the breadcrumb, the datetime was different than in the form if the user had another timezone than UTC.

This rev. is related to 27d8cb84, but is for the 8.0 api

We are aware we introduce a tiny change of API (method signature change), which we normally prohibit, but considering the really low level of the method, the fact it is probably not ovveriden by any other modules and the fact there is no cleaner way to correct this, we are making an exception.