Registrars updates

Since its first implementation around the end of 2019, the `Registrar #1` (which is now a set of registrars), has evolved a lot. This article looks back on those evolutions as well as at the current stand. I will go through the challenges I ran into and how they have been solved. This includes a few dedicated web services, the use of a BPM engine as well as new functionnalities recently added to Polkadot, Kusama and the Westend chains.