simplify logic by changing the dao function to require a user id and a
webauthn cred id. Note that this changes the response from a 400 to a
404 if the cred is for a different user than the supplied id.
give a minimum length to the text fields in POSTS to create/update a
credential to avoid surprising unexpected edge cases involving empty
string names etc.
Also fix tests:
First add init file so the tests are found correctly, then update
the tests after we stopped serialising webauthn
registration_response.
added some simple validation to the delete endpoint for sanity, but
generally my assumption is that more validation will happen on the admin
side.
noteably im not checking whether the credentials are duplicated, nor is
there a uniqueness constraint in the database - I'm not sure if the
credential blob will always reliably be equivalent, and I believe the
browser should hopefully take care of dupes.
