ldapadmin - Pass a list of required fields to new account form

TODO:

* define the list in config/
* use this list in server-side validation
ldapadmin - list of required fields for new account form defined in config/
ldapadmin - give possible values for required fields in new account form
Cut the fields list into two lists: real fields and "hidden" fields
New file that will centralize the validation methods for forms
Move requiredFields management from NewAccountFormController to Validation
add minimum set of required fields
isRequiredField will return a boolean, not a String
Test if "uid", "surname" and "firstName" were filled, only if they were required
Set session attribute instead of model attribute

This allows to get these attributes also after a validation error, see l.130
Do not test if email, password and confirmPassword are filled if not required
add validation for four optional fields, depending on the config
Remove useless file
avoid java.lang.StringIndexOutOfBoundsException if string is empty
Better comment
create a static variable for fields list
remove four unused fields
add server validation for six optional fields
order alphabetically
set required fields to jsp page