Keys remember whether to use deprecated syntax

and the deprecated key handler registers itself with the base key type
LMS-2752
If we're removing `make_usage_key_from_deprecated_string`, then I assume we're not updating edx-platform to point to this version of opaque-keys until we've changed all the uses of `make_usage_key_from_deprecated_string` within the edx-platform repo?  (Will we just be using `UsageKey.from_string()` everywhere?)