Create and use a new "HasURL" role

[davorg]

The recent canonical URL fix highlighted that URL generation logic is spread across the application. Consider moving URL construction into model objects via a HasURL role.

Something like this:

role Succession::Role::HasURL {
  requires qw[identifier];

  method prefix { return undef }

  method url {
    return join '/',
      '',
      grep { defined && length }
      $self->prefix,
      $self->identifier;
  }
}

And then, in the Person class:

method prefix     { 'p' }
method identifier { $self->slug }

Might need a Date class too:

method identifier { $self->canonical_date }

And, perhaps, we can also use it for the reference pages.