Clean up + re-strucuture of person validator test class

* refs #17403
Add validation for period's dates in person form

* From and to of period must be valid, i.e. a date
  like the 31st February is not valid.

* refs #17403
Fix false positive validation of representative in person form

* refs #17404
Adjust styling of login page

* Login failed error component should be
  Bootstrap styled instead of custom styling.
Add custom 404 page

* refs #17769
Allow only test user to login through test user details service
Change special leave icon color to yellow on overview page

* Background color of special leave icon on overview
  page should be analogue to the holiday's background
  color because in the calendar both types of holidays
  are displayed in the same color.
Adjust list tables styling

* Enlarge font size and remove usage of `h5` tag.
Adjust error page to new styling

* Error page is styled now analogue to
  404 page.

* refs #17769
Show 404 page on 400 error

* refs #17769