Build first-pass admin page for #492.

Enable Django's simple built-in admin page for Partner Species. This
should enable simple editing of partner species rather than using the
bulk interface.  Specificially it can be used to:
- Filter the list of species by partner site
- Search for a particular species or partner site by name
- Remove Partner-Species relationships.
- Add a new Partner-Species relationship.
- Edit the details of a Partner-Species relationship, such as Species
  Page Header, Species Page Blurb text, and whether the species appears
in the Simple Key.