imp: Allow admin to change the LDAP identifiers

[marien-probesys]

Related issue(s)

#168

Changes

• in admin, forbid changing name, email and password of users if ldapIdentifier is set
• add an input to change the ldapIdentifier
• when synchronizing, search users by their ldapIdentifier instead of email

How to test manually

• go to the settings > users
• edit charlie user → check that you can't change email, name or password
• empty the ldapIdentifier input → check that you can now change email, name and password
• set "charlie" back to the ldapIdentifier and run ./docker/bin/console app:ldap:sync
• check that the information are back

Checklist

• code is manually tested
• permissions / authorizations are verified
• interface works on both mobiles and big screens
• accessibility has been tested
• tests are up-to-date
• locales are synchronized
• copyright notices are up-to-date
• documentation is up-to-date (including migration notes)