Add migration to IPS 4.3

[freezy]

The new 4.3 release supports a full-fledged OAuth2 server. It has more features and will be maintained by Invision, so there should be a way to migrate ips4-oauth2-server users to the native version.

I've tested ips4-oauth2-server with 4.3 and it seems to work, so it's not an urgent issue. However, if you don't tell me you're already on it I'll start working on a PR.

[wohali]

Would love to see the PR - please contribute!