rename webauthn auth functions

_complete_webauthn_authentication -> _verify_webauthn_authentication

This function just does verification of the actual auth process -
checking the challenge is correct, the signature matches the public key
we have stored in our database, etc.

verify_webauthn_login -> _complete_webauthn_login_attempt

This function doesn't do any actual verification, we've already verified
the user is who they say they are (or not), it's about marking the
attempt, either unsuccessful (we bump the failed_login_count in the db)
or successful (we set the logged_in_at and current_session_id in the
db).

This change also informs changes to the names of methods on the user
model and in user_api_client.

app/notify_client/user_api_client.py

@@ -126,8 +126,9 @@ class UserApiClient(NotifyAdminAPIClient):
             raise e
 
     @cache.delete('user-{user_id}')
-    def verify_webauthn_login(self, user_id, is_successful):
+    def complete_webauthn_login_attempt(self, user_id, is_successful):
         data = {'successful': is_successful}
+        # TODO: Change this to `/complete/webauthn-login`
         endpoint = f'/user/{user_id}/verify/webauthn-login'
         try:
             self.post(endpoint, data=data)