Integrate with the API for adding and getting webauthn creds

This links up the `get_webauthn_credentials_for_user` and
`create_webauthn_credential_for_user` methods of the user api client to
notifications-api.

To send data to the API we need strings to be unicode, so we call
decode('utf-8') on base64 objects.

Co-authored-by: Leo Hemsted <leo.hemsted@digital.cabinet-office.gov.uk>

tests/app/main/views/test_user_profile.py

@@ -19,8 +19,10 @@ def test_should_show_overview_page(
 
 def test_overview_page_shows_disable_for_platform_admin(
     client_request,
-    platform_admin_user
+    platform_admin_user,
+    mocker
 ):
+    mocker.patch('app.user_api_client.get_webauthn_credentials_for_user')
     client_request.login(platform_admin_user)
     page = client_request.get('main.user_profile')
     assert page.select_one('h1').text.strip() == 'Your profile'