Fix the new Django admin user edit form's password change button

[jbradberry]

At some point in the recent past
(https://code.djangoproject.com/ticket/34977), Django changed the user edit form away from showing a link to the password change page, into a button. This relied on a call to get_context(), which this custom password widget wasn't invoking.

[jbradberry]

old version:

new version:

[jbradberry]

The reason for the check for the presence of the 'value' key in the context['summary'] entries is because those are only populated when Django puts together the ugly hash, etc stuff when a valid password is set, e.g.:

[
  {'label': 'algorithm', 'value': 'pbkdf2_sha256'},
  {'label': 'iterations', 'value': 1000000},
  {'label': 'salt', 'value': 'HjmQxM****************'},
  {'label': 'hash', 'value': 'NCFBAA**************************************'}
]

ref: https://github.com/django/django/blob/5.2.5/django/contrib/auth/forms.py#L44-L59