Drop first/last names in favour of a single full_name field

opentech/apply/users/forms.py

...tech/apply/users/migrations/0004_drop_first_last_names.py

opentech/apply/users/models.py

...tech/apply/users/templates/wagtailusers/users/create.html

opentech/apply/users/templates/wagtailusers/users/edit.html

opentech/settings/base.py