Rename "key" to "secret" for consistency
Rename "key" to "secret" for consistency

While "key" is the term used by the JWT library, all the rest of
our code - the ApiKey model, the Python client - all use the term
"secret" instead. Although "secret" is less precise than "key", it
does help avoid confusion with (api) key (as a model object).

alphagov/notifications-api,alphagov/notifications-api