Don't require setting first name or last name

[nphilipp]

Storytime

As someone who doesn't have a first(*) or last name, or someone else who is interested in their privacy,

I want that Noggin/FreeIPA doesn't require me to set a first or last name,

so that my real name is faithfully represented, or (as the case may be) I don't have to divulge my identity or invent something.

Background

• The existing FAS2 system allows user to register without supplying a first or last name.
• Apparently, FreeIPA uses a generic LDAP object type which requires that the o_givenname and o_sn fields are set.
• The fas2ipa migration tool therefore uses <fnu> and <lnu> as placeholders to deal with the existing accounts that don't have either field. I've filed fedora-infra/fas2ipa#12 to address that these placeholders aren't really speaking (just in case they leak to a user).

Acceptance Criteria

• [ ] Noggin lets users leave first or last name unset (see PR #329), and
• [ ] either translates back and forth between that and the agreed upon placeholders (the same fas2ipa uses),
• [ ] or FreeIPA allows leaving these fields unset/empty downstream.