Enable eager registration data validation after unfocusing a widget

[rbaltrusch]

Currently, the full user registration page uses lazy validation (i.e. all validation happens when submitting the data by clicking the register button). This is suboptimal, because it may confront the user with a number of issues with the data at the end of data entry, rather than during entry.

See the currently lazy validation in action here:
https://user-images.githubusercontent.com/41642326/144753437-2b5c3867-1d3a-4066-9f06-2160737edde8.mp4

Consider keeping lazy validation if the widget has not been changed yet and enabling eager validation (i.e. validating every change) as soon as the widget is unfocused for the first time. This way, users will not be confronted with many validation errors at the start or the end of data entry.