Convert PasswordFields to GovukPasswordFields

Converts them directly in the following forms:
- LoginForm
- ConfirmPasswordForm

Changes the password function to return
GovukPasswordField instead of PasswordField which
effects the following forms:
- RegisterUserForm
- RegisterUserFromInviteForm
- RegisterUserFromOrgInviteForm
- NewPasswordForm
- ChangePasswordForm

It also updates StringField on RegisterUserFromOrgInviteForm
to GovukTextInputField

Also includes changes to templates that use this
form and associated tests.

tests/app/main/views/test_register.py

@@ -458,8 +458,8 @@ def test_cannot_register_with_sms_auth_and_missing_mobile_number(
 
     assert response.status_code == 200
     page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
-    err = page.select_one('.error-message')
-    assert err.text.strip() == 'Cannot be empty'
+    err = page.select_one('.govuk-error-message')
+    assert err.text.strip() == 'Error: Cannot be empty'
     assert err.attrs['data-error-label'] == 'mobile_number'