Add persons/add/ and persons/{id}/edit

Following the pattern set by 0957f12d (Create and edit sites using
django generic views, 2014-12-09, [1]).

The pk_url_kwarg field [2] avoids:

  AttributeError at /workshops/person/7458/edit

  Generic detail view PersonUpdate must be called with either an
    object pk or a slug.

The '__all__' value for fields automatically uses all the forms
fields.  The Django docs point out that this could be a security
problem if you add a secret field to a model and forget that it will
be visible and editable by anyone with permissions for that form [3],
but I don't expect we'll be needing secret Person information and it's
good to stay DRY.

[1]: https://github.com/swcarpentry/amy/commit/0957f12dfb830013b190d7e3a1395fdfea6d1108
[2]: https://docs.djangoproject.com/en/1.7/ref/class-based-views/mixins-single-object/#django.views.generic.detail.SingleObjectMixin.pk_url_kwarg
[3]: https://docs.djangoproject.com/en/1.7/topics/forms/modelforms/#selecting-the-fields-to-use