Figure out future of `/retest` app

We made a load of changes to the retest app recently to work with Envoy's new CI framework

This may have broken some users as the logic is now more tied to Envoy's checks and essentially the `github/checks` action which has related logic

It was my intention to fold the action into that one

I reckon we can support a generic `/retest` one way/other so i guess its a case of figuring out how/where to maintain the code